My current marina has tons of room for parking. So, no parking fee.
The marina that the previous owner of our current boat used, had very little availability for parking, and so the seller's broker who was trying to run his business there gave us special instructions of the best place to park and hoof it in. So, you guessed it, in addition to one's seasonal slip fee, there was a parking fee for the boat owners there. There was also a restaurant/bar on site. Who knows what those patrons did?! That establishment was closed due to Covid last year when we were making our visits to buy the boat. So, I think it is very simply a matter of market forces / supply and demand. You cannot fault anyone for making an honest buck in this country.