Well, here's my take on this one:
- "Munus" wasn't necessarily for the community, it could also be by the community.
The "cip" is the root of "capere," but not necessarily bearing the same meaning.
A source of capable is "capax," (able to hold much), which we can see in capacity, what something is able to hold.
A Roman municipum was a city whose citizens had the privileges of Roman citizens but was governed by its own laws.
Putting these together, I come up with the idea that a municipality is a town large enough to make its own local laws.
Much like US cities. Subject to federal and state laws, but also allowed to make their own in addition.
Information gleaned from http://www.etymonline.com.