Just as someone earlier figured that some may over-state the number who get up because it is annoying (which I don't doubt), I think you are seriously under-stating it to say it is 10%, because lots of people get sucked into the movie and probably don't pay attention to it when people do get up. I have noticed it, just like I notice annoying people talking on cell phones or amongst themselves, though some people I've gone to movies with don't.
People are constantly getting up and coming back in. It is staggered throughout the film, so it doesn't seem like a big chunk, but I bet it usually is. None of this speculation matters much, though, because it is all based on anecdotal evidence, and it all is completely irrelevant to the suggestion that theaters could make money by giving away free popcorn.