If I had to guess, since M14's, like FN-FAL's, came from the factory both as semi as well as full auto flavors, the 14 fell thru the cracks .... Now the last time the CONSERVATIVES were in power, under Kim Campbell's leadership (if anything it should teach every Canadian a lesson about the dangers of having a (granted, unelected) western PM), they "sorted the whole mess out" in a way by declaring every "converted-auto" prohib .... Yeah, I'm one of those guys that trust politicians of any flavor about as much as I trust a starving, wounded, black bear whom is coming right at me .... It just HAS to be friendly, like a dog, right ??? I'm being gentle here, my opinion of the very concept of a political "party" is a whole lot less "understanding" ....
And since there was no physical difference between F/A and S/A M14's, except for adding the fun switch (which didn't come from the factory but could be added later on by anyone who felt like it and got hold of the parts), it became politically convenient to lump the whole mess into a single category ....
Lemme ask you this : Why are there 12(3) Steyr AUG's out there, but no 12(4) despite the fact that there was such a thing as a semi-auto version ?
If you'd think that 12(4)'s were pretty much all seized and destroyed, without compensation, except for a select few who'se owners went to court over the matter and won, you'd be right .... I know of some people who got their 12(3)'s "grandfathered" and got their 12(4)'s seized, putting them tens of thousands of dollars in the hole in the process ... Sometimes it just plain sucks when your shooting buddy is the local police chief so registry or not, they know you have it ...