Greentips, the reason the AR-15 is restricted is because the rifle is specifically named as such by OIC. Regardless of its commonality with the M-16, or converted autos or any such stuff, the reason it is restricted is because it is restricted by name by OIC.
Most people don't realize that after the Ecole Polytechnique massacre, the Kim Campbell government went through a copy of Guns Digest and anything they thought looked scary in appearance was listed and then that list was added to Orders In Council as being restricted or prohibited. It is for this reason that the AR-15 is restricted (because it was in the 1990 edition of Guns Digest), and the Swiss Arms Black Special and Robinson XCR are not (because they didn't exist in 1990). It is also why the H&K G11, which in reality does not exist in private hands anywhere, is prohibited (because there was a picture in the 1990 Guns Digest).