Casino money are never for FREE, it always come with some kind of wagering requirement and they know most people will simply use that to gamble more, so they get it all back in the end.

Yes, the "Slot pick" requirement is unfair and I would avoid playing those Slot, because you will lose more than what you are gaining from the bonus.

In the end, I use the casino money to wager up.... not to waste on Slots with a shitty RTP or Variance.