But you do say that you visit them a lot, and they rarely visit you.
Maybe they feel you should pay a little towards your stay as it is fairly frequent, maybe they think you are taking advantage.
Lets face it, while you are there you are probably using some of their belongings (loo roll, tomato ketchup, heating, lighting, telephone, hot water, towels, that sort of thing) and it all mounts up.
Maybe take a food and goodie hamper next time you visit (with some loo roll in it) that would be a nice gesture.