[wiml]

I think it's because it alters the moral context of the act that you're going to take anyway. If I pick up a piece of litter on my own initiative, that's an act of virtue. If I pick it up after a petty authority tells me to, that's an act of subservience. People get upset at the existence of PSAs because (in this view) they rob them of the ability to act virtuously. Which is not at all a small thing!

Yes, emotional maturity can help, and it also depends on perspective (whether I identify with the author of the PSA). But when the situation gets more complex than picking up litter, I think people forget that this dynamic exists and is important.