The only downside is to insurance companies, many benefit from these fires from owners, to fire companies, to those who build new super yachts to those supplying all the components. And the marine salvage business. See many would be out of work otherwise.
As long as they are careful not to kill anyone, and keep secret the owners setting these fires get rewarded with cash and new yachts.
Selfish though for them to set fire to their boats at docks. Maybe the other guys superyacht is not yet ready for the fire.