I do pretty much the same way you do. I warn them ahead of time, the worst possible scenario is no photographs. I also accept no payment although I do accept modest token of appreciation if offered, but not cash. This is my hobby. I don't want money involved in it.
I typically decline to attend wedding with full compliment of my cameras unless the hired photographer explicitly consents to it. If I do it, I want to do it right and to do that I need complete freedom which may interfere with a hired gun. I really don't like to do things I don't want anyone else to do to me. Like you, I may take simple stuff and take limited numbers. If I can get some worthwhile image, great, if not, I was never there to take a photograph in the first place.