...They collect valuable behavioral information.
Do you really think even the reward points for the occasional free popcorn compensates you for the ultimate value of that information? How long do you think it will be before someone figures out how to use that information in interactions you don't even suspect are intended to be just as diverting (as already indicated here); thus keeping you properly channeled into the desired information stream as much as is possible.
Very profitable for them in the long run. Very profitable indeed. Not that they're likely to keep it up indefinitely mind you, but a carrot like that dangling before most of us would be pretty hard to ignore, even with running towards a precipice.