I think it's pretty different. In the candy case, you're just not buying something that you might or might not have bought from the theater, but the theater can still sell that candy to someone else. In the popcorn instance, you not only aren't buying something from the theater, you are getting an actual product from the theater that you should be paying for, for free. And product they can't sell to someone else.
In addition, unless you shoplifted the candy, you still paid for it, they appropriate price wherever you bought it. You didn't steal anything, you just broke a rule. You didn't get anything for free.
Bringing candy is unethical, but the popcorn example is flat out stealing a product. The first is like going to Best Buy and taking up a salesman's time to quote a TV, knowing you are going to take the price to Costco for a price match. The second is stealing the TV.