Very interesting. I'm not exactly sure why the union vetoed it. Perhaps they felt like it would cause many QBs who would otherwise have jobs on the 53-man roster to instead be carried on the Practice Squad for far less money? So it promotes higher pay and job security for veteran QBs...? All I can think of.
I don't think it really affects the Pats since they're likely to carry 3 QBs regardless, although I guess theoretically they might've wanted to declare Milton a game-day inactive and make Zappe the "emergency 3rd" from the PS or something. Now, they'd need to have Zappe on the 53 man roster instead but otherwise functionally the same process would apply. In all likelihood, Zappe probably won't be here and Milton will be the "emergency 3rd" on the roster regardless.