Sure, on strict legal principles it's wrong for someone to to pick up and use his private property. But it's equally true that the mooring owner has appropriated public space for his own use. And on strict legal grounds, the Jackwad also has options.
In our area legally placed moorings are allocated by the Harbormaster. The space is not "appropriated" by the mooring owner.
If there is a Town Harbor Plan in effect--at least here in CT--the Harbormaster (a creature of the State of CT)--is required to observe the Harbor Plan. The point of the Harbor Plan is like that of a Zoning Plan--to provide a set of considered policies in the public interest. An approved Harbor Plan has the force of an ordinance, where applicable.
The Harbor Plan allocates areas for commercial and private moorings and identifies a policy for assigning mooring permits, including a posted waiting list when the mooring field(s) are full. This means that moorings are allocated with consideration of the conditions of the mooring area and the type of boat on a first-come, first served basis.
This process works something like parking on a public street, but the difference is that the person who got there first not only occupies it at the time, but retains it on a reserved basis.
The intent is to provide for a safe, orderly anchorage tailored for boat swing and moderate storm conditions by the use of mooring tackle that is more substantial than typical cruising anchors.
In our area, we expect transients to use the designated transient anchorages--not the mooring fields, unless you are picking up an authorized rental mooring. Private moorings are not for rent.