I have noticed that some people just do this, and some just don't.
Some people feel entitled to help themselves to piles of food to take-away as they have paid for brekkie, and feel that it is expected and normal that everyone helps themselves. They are shameless about it as they feel it is something they have paid for and is totally normal.
Other people feel that it is weird and wrong to steal food from breakfast and secrete it out of the place. Cheap and rather minging. Embarassing.
i don't know where/how this difference arises - in my friendship group some are the former and some are the latter, with no difference in class, education blah blah.
maybe some people are just tight?
Dunno.