As a former contractor, I saw the occasional bag full of milk cartons. I find it weird. I don't think it's worth a crackdown or treating the employees like babies, though.
It is true that no matter how much you pay a person, they can still be cheap and "exploit" a resource. I don't think it's important enough to worry about on the whole.
Would you take home a bunch of sodas if you were going to have a party that weekend?
What if your party guests were exclusively other employees?
What if your party guests were partially other employees, their families, and some unrelated families?
Would you drink an employer-provided soda at home if you weren't thinking about work?
What if you were at the office and not thinking about work?