Uh, I sometimes do but my husband does it for me as well. Fortunately my husband only asks to massage his back and hands. I don't think he likes me rubbing his feet but I have done it.
There's nothing wrong with your husband asking you for a foot rub as long as he's doing something nice for you in return. But if he isn't, that really isn't fair to you. He shouldn't demand for you to do something, it should be asked nicely.
Also, he should be considerate in that he should wash his feet before asking you to rub them. If you are willing to rub his feet, ask him politely to clean them first.