I disagree to a certain extent, with that, Mints. The “Street Sweeper” and Tec-9 in particular were not designed for nor of any real use for a true military application.
Of the firearms listed by Sofa, only the AUG, AR-15 and some of the AK variants could accept bayonets.
As noted earlier, the “grenade launcher” was a nonissue to start with (as in most civvies could not or would not go through the trouble to obtain a strictly-controlled weapon and even more strictly controlled and very expensive ammunition) and so it’s inclusion is suspect.
It’s my impression that buyers picked up the expensive but still legal 37mm version largely for the cosmetics- it makes the AR-15 look like a military M-203/M-16 combination.
And that ties in with the rest: Having a bayonet on a rifle is a feature more cosmetic than utilitarian. As is a pistol grip (vs. a non-pistol-gripped or thumbhole-stocked version of the same gun) the ‘flash suppressor’ (especially considering that other rifles’ “muzzle brakes” weren’t affected, the the concept and operation is essentially identical) and the folding stocks (most affected rifles even with the stock folded are still many times larger than most pistols, so concealability wasn’t an issue.)
In other words, it was the Law Against Ugly Black Guns.
As Catsix noted, for example, the AR-15 is a semiautomatic rifle that shoots .223 Remington and takes detachable magazines up to 40 rounds capacity. The Ruger Mini-14 on the other hand, is a semiautomatic rifle that shoots .223 Remington and accepts detachable magazines up to 40 rounds capacity (but has a wooden stock as opposed to a black plastic stock.)
The AR-15 was named specifically as banned, the Ruger Mini-14 was specifically named as excluded.
I suppose there are other answers, but I interpret that as being a law against nothing more than cosmetic features that somehow look evil and intimidating to those who don’t know any better.