For example there is a local yacht club that has about 15 destination moorings. They are all placed with town approval and are properly regulated and inspected. Strictly speaking they belong to individual club members who apply to the towns for the mooring permit.
As far as other moorings go, you never know what is underwater. A few years ago when they dredged Bass Harbor all the moorings were removed. Quite a few of the moorings were in very poor condition. A number of them failed when they were being removed because the chain was wasted away.
Finally, a normal mooring in a town harbor costs between $1,500 and $3,500 (depending on depth and weight of tackle) to set and $400+ per year (on average) to maintain. If you use someones mooring you are effectively stealing from them. Furthermore, here on MDI, it takes years on the waiting list to get a mooring.