I guess it all depends on what the local rules are for mooring use. I can only think of a couple of ports where every mooring is available for anyone to grab on a first come first serve basis. But it's clearly spelled out that it works that way. I've been to plenty of places where there are specifically designated moorings that are first-come-first-serve, grab what you want, but it's always only applied to some clearly designated set of moorings. More frequently, the harbor master will assign you a mooring from a pool of transient moorings plus unused private moorings, but you ask first and are granted use of a mooring.
I think it's rude and presumptuous to assume you can go up to any mooring and use it without some explicit indication that you can. Same with a dock.