Josh, I'd comment on your IQ, but it would be a waste of time. Rather than saying, "Why do they..." and "I think..." just ask for an explanation. There are BATFE regs one can look up and read. I have, more than once, but for all you grasshoppers more interested in mouthing questions than a tad of looking, any weapon over 50 cal is a Destructive Device (DD) and a 2-inch bore knee mortar is definately over 50 cal. Unregistered DDs and MGs could be registered back during a two-month period, Nov-Dec in the 60s. After that two-month period there is no way us average folk can register an unregistered MG or DD. A person with an ATF Manufacturer's license can turn an unregistered MG or mortar legal, i.e., register it with some "magic" process, remanufacturing it, but the in's and outs are beyond me, maybe my buddy of wit and wisdom knows.
Today, a REGISTERED live or dewat mortar can be transferred. The transfer fee for a registered live one costs $200, there is no charge for the registered dewat. (And a dewat registered back in the 60s period did not have to have the hole drilled in the barrel. A barrel dent and a few pieces removed sufficed to register it as a dewat, and it, like the MGs with a brass or lead plug in the barrel, could probably be dewatted in more than one way.
A dewat mortar (or MG) can be "rewatted, made "live," by filling out the paperwork and paying the $200 fee. Why anyone would do so with a mortar is beyond me, a MG is different in that you can shoot it. But with either it could be asking for trouble, taxed or confiscated in the future.
If you run across a T-10 or T 89 for sale that is not registered,, i.e. and illegal one, you can purchase if you so desire, but have the owner remove the barrel and leave the base/"stem" with relatives, neighbors, etc. before money changes hands and you assume ownership. You can legally own the barrel without the remainder of the weapon, or own the base/stem without the barrel. It's a little "gray here, I have a letter from ATF that says what I just said, you can own the barrel if the base is not readily available or you can own the base and make a dummy barrel as long as the original barrel is not readily available. What constitutes "readily available?" If you have possession of the barrel and another has possession of the rest of the infernal machine, what would ATF say...??? The "Don't ask, don't tell" rule applies here. Just remember the person with th mortar is an "unconvicted felon" and you are dealing with him. I would not be suprised to hear that "G-men" would try to peddle a knee mortar to entrap one of us so I'd suggest you know the seller.
Once you have the barrel in hand trot off to your local machine shop and have a groove milled in the back, it does not have to be that eagle-size bird house hole that are drilled in many 89s and 10s. The area of the groove should equal the area of the bore and it should extend from near the base to near the top. This is where your 5th grade arithmetic comes in, i.e. calculating the area of a two-inch diameter circle. (I'll admit I had to think about that.) Once the calculation is made ( I know how, but you need the 5th grade arithmetic review) determine how wide and long your groove needs to be, i.e., the area of a rectangle extending along the back of the mortar from 'near" the top to bottom that is equal to the area of your two-inch circle. Better too large than too small to keep "Uncle" happy.
Then have an obstruction, it doesn't have to be a bar, AN OBSTRUCTION, welded in the barrel, anything to keep a live round purchased during the K-Mart Blue Light Special sale from being dropped down and fired about two feet as all the propelling gases exit through the groove. Once the groove is milled and the obstruction welded the mortar is considered "destroyed" (ATF terminology) and you can then go back and pick up the mortar base and assemble/reassemble. Before having the obstruction welded in the barrel you might consider dropping a 'dewat' round in. The obstruction will keep it in place and a person can see how the round rests on the firing pin "worm" and how the knob on the side will adjust the range. This amazes most folks who think that an 89 or 10 is just a section of pipe fitted to a base.
The legal aspects of dewating both a MG receiver and a knee mortar are covered in a three page letter from ATF published in a recent BANZAI. This is the letter that says you can not fire a solid "plastic" shell from a legally registered mortar using a spoon-full of black power in a removable container in the plastic shell base. I use/USED the lower 25% of a chopped off brass shot gun shell to hold the powder. While waiting for a reply to my above mentioned letter I spoke with the Assistant Director of the ATF's Firearm's Technology Branch (?) about firing such a projectile. He said he "saw no problem." The people working under him apparently did and wanted to protect their fanny in case a solid plastic shell was fired at and struck one of "officials" in Disneyland East (Washington D. C.)
Over 15 years ago I asked a Disneyland East ATF agent about using a groove instead of the hole to "destroy" the mortar, the agent I spoke with, a nice fellow, said he saw "no problem," but when I asked him to put it on paper he declined, CTA (cover thy ass!). I was stunned that it was OKed this time. Probably took several hundred hours of meetings, argueing the pros and cons, and trips to K-Mart to observe the Blue Light Specials to decide that a groove was equal, maybe better, than the hole.
And as far as Canada allowing live, unaltered knee mortar ownership, they are idiots, Everyone knows that the knee mortar is the terrorist's weapon-of choice. Or maybe they don't have Blue Light Specials at their K-Marts? And now kiddies, get out your pencils and paper, it's test time!