Simply put, it’s a sign-up bonus where whomsoever signs up at that specific establishment is awarded with free money. It’s something many casinos don’t do for obvious reasons. That being said, there are still some who do this making it clear that they have so much money to burn that these sorts of bonuses won’t hurt them.
That or they have a fair bit of clauses to them to ensure that you are spending more than what you’re getting back.