I also hear that same rebuffal when people are declining a request, offer or invitation. "Hey are you free to go out for dinner on Saturday?" "I can't...[insert plausible sounding excuse here]." People use this form when they don't really want to do something, so instead of saying, "no, thank you", they feel compelled to save the feelings of the other person with well-meaning, but nevertheless false courtesy. I'd rather you just say no, so that I understand your preferences and maybe make an offer more to your liking. And if it's just me you don't like, the truth saves a lot of time and aggravation. That message can be delivered with real courtesy.