I totally agree with the first in, last out concept. However, that does not give the first boat in a mooring field the right to dictate what goes on at all of the mooring balls around them. After all, they are just renting the ball for the night, they do not own it. The boat in front simply had no right to tell the later boat in that they could not use the mooring behind them. The captain of the first boat in, if they felt it was truly unsafe, while the captain on the later boat in, also paying a fee to the mooring owner (who presumably is a professional with the requisite experience (the boat captain, not the mooring owner)), has a right to use that mooring if in his judgment it is safe to do so. If the captain/owner/charterer of the first boat determines that the situation is unsafe due to the arrival of the second boat, they can either attempt to convince the owner of the mooring not to rent that ball for the night (unlikely), or they can move. Hopefully at some point common courtesy and common sense win the day.

Totally different situation from anchoring, just in my humble opinion.