The reason why online gambling operators enforce caps on the maximum winnings players can cash out from their bonuses is quite obvious. They use these caps as a form of damage control to prevent bonus play from affecting their overall profitability.
Some bonuses, especially those that do not require any real-money deposits, enable players to win some cash with zero risk for their bankrolls involved.
As good as it sounds, online casinos are like all other businesses, and turning a profit is their primary purpose. Allowing players to withdraw all their free-play winnings straight away could affect the operators’ deadline and cause them to lose money.
Customers would simply redeem the free cash, play with it, and withdraw whatever they have won, without returning to wager any real money again.