Why do online casinos give away free money?
Certain games are weighted differently
Things are not as simple as just betting your bonus a set number of times. Casinos will apply weightings to certain games. Here is a typical example:
Slots/Scratch-It Cards: 100% weighting
US Roulette/Casino Poker Variants: 25%+
Video Poker: 5%+
Some games could be excluded altogether. Examples include French roulette, video poker...