Volunteering to maintain a project literally does create entail accepting duties, that's what taking on the role of maintainer entails. They are of course free to give up that role at any time, but those duties exist while that role has been adopted.
You can't really say someone has a "duty" without also implying that they have a "responsibility," and thus liability if they fail to execute those duties properly. I don't see how this fits at all for a volunteer. Very few people are going to volunteer for no pay if they're taking on a risk of liability.
Maybe you mean a civic duty? That would make somewhat more sense, but the problem is that there’s no objective standard against which to test performance. It’s completely subjective and will be forever argued—much like this thread. :-)
