Carmike considered this; they even put the policy into place - only to retract it a mere few hours later. They did not deem it worth the trouble or potential backlash.
It's also worth noting that the memorandum sent to general managers did not involve removing (or even checking for) outside food / candy.
So, again, it bears repeating: they aren't taking your candy away. Honestly, do you think the staff even WOULD give a shit if they saw your Twix? These are mostly kids being paid minimum wage. They're told to upsell to you at the concession stand, but are not going to go on a power trip over outside candy - those kids deal with enough assholes every day (food service + retail in one building, I'm not brave enough, I'll admit) to want to deal with someone flipping out because the Constitution entitles him to his fucking Jujubes.
Besides, having a good friend who works as a theater GM, outside food has NEVER been an issue as long as it is not disruptive (loud packaging, smelly, whatever). Popcorn and soda are the big margin items anyway, and most movie-goers buy both, especially with theaters bringing in Coke Freestyle machines and self-service popcorn to make the experience a bit more enticing. The few folks "sneaking" in candy don't really impact the bottom line any more today than they ever have.
Regal's idea isn't a true deterrent or solution, as many of you have said, so I roll my eyes at them presenting it as such...but, hell, if it makes someone feel safe, or maybe DOES prevent one psycho from trying the "next big thing" in theater destruction, then some kid glancing in my girlfriend's purse for literally a second before moving to the next person does not bother me.