Originally Posted by
reimero
It's ambiguous, but arguably falls under "federal law requires passenger compliance with posted placards, lighted information signs and crew member instructions." It boils down to how much of an issue the flight crew wants to make of it, since this does constitute a "crew member instruction".
Bolding mine - and this would exactly be the case. I suspect if someone started munching on some trail mix 10 rows away and there were no ill effects on the passenger with an allergy, the FAs might not do much. But if someone started slurping down spoons of peanut butter right next to the allergic passenger and they began to have a reaction, making the announcement gives the FA the ability to say they had indeed provided instruction that was not followed.
Much of the "firm" instruction is quite obvious (e.g., we can't move unless everyone is seated; don't use an e-cig; you need to hang up the phone now). But there are lots of crew instructions that are regularly not followed completely (don't use the lav while seat belt sign is on; enjoy the rest of your day; etc.). It's certainly almost always advisable to follow crew instruction, but the extent of enforcement is certainly up to how they actually treat each situation.