Why it's legal to put one of those wire stocks on a Glock pistol and it doesn't classify it as an SBR but it's illegal for me to put a folding stock on my Calico M-950 w/a 7.5" barrel unless I register it as an SBR?  I'm really confused.

...WELL...NOW THAT YOU BROUGHT IT UP.......

Erm, it's NOT legal to put a stock of any kind on a glock pistol...unless it's a post-sample full-auto glock and you're a LEO.

Unless I'm missing something.

ya, i don't think a pistol can have a stock.
there was a fairly big go-round with T/C and the ATF over one of the contender setups that could be configured as a carbine, 16 inch bbl and stock, or a pistol with no stock and a short barrel.
the problem was that the fuds considered it possible to assemble an SBR by using the stock with one of the shorter barrels included in the kit.
so, if i am recalling it correctly the only way to put a stock on a glock is too either go NFA or add one of those carbine top ends.