The AR 15, if manuf. properly as a stand alone A R lower is still legal, however a lot of lowers manuf. in the U.S. have the milled out clearances & frame holes to allow installation of M 16 sears & trigger groups (R&R fun parts), these lowers are completely legal stateside as a U.S citizen (in an appropriate State) can either buy the appropriate tax for F.A. or just build it to A.R. standards...this is completely legal for them as it is what they have in their hands...not what is possible to have in their hands as it is here (only with firearms I might add). These are the lowers that they are seizing as lots of them were imported before the internal policy change was made and importation of them was un-controlled. I hope it wasn't but the possibility that your lower sat on an importers shelf since before the border crackdown still remains (or you may have purchased it before policy change as well).
The so-called 80 %'ers attract a lot of attention for the above reason as most of them are home-built to available inter-net plans that include the offensive cut-outs and most unsuspecting machinist (either home or commercial) up here just cut to the plans offered, not realizing that some cuts aren't required for a legal A R lower.
As I said, I don't agree with their position but that ain't worth even a cup-o-coffee anywhere LOL. I do agree with the Drunk that a search of CanadianGunNutz.com will garner way more pertinent info than what I can pass on.