There are four possible reasons the club won't be allowing flasks into the Amex:
1) Commercial. PB has addressed this.
2) Explosives. I presume if the club genuinely think a flask could explode then I'd expect them to have a full "Hurt Locker" set up along with a bomb-proof container/storage...
Will there be flask carrying protests?
What's going to happen to all the seized flasks?
Will you get a unique i.d. number to allow you to reclaim it on your way out or will there be flask bins so the club can take them to Richardsons for ££££ ?