I think I acquired this sort of habit through being self-employed: there are lots of things that need to be done for the business, and there isn't enough time to do even the important ones, so I feel guilty about times when I'm not working but could be. My usual, "well, they're at least getting what they're paying for and what was promised" doesn't work when I am them. I wonder if being involved with free software that many others use could cause a similar effect, through knowledge of the further work you could be doing on it.