Casino bonuses are abused in a number of ways. Firstly, some players will sign up for multiple accounts and take advantage of the welcome bonus multiple times. Secondly, players may try to withdraw their winnings without meeting the wagering requirements. Finally, players may also use betting systems that circulate money through different accounts (usually family or friends) in order to taking advantage of bonuses.
All of these methods are technically abuse, but they’re all quite common. The best way to avoid being abused by a casino bonus is to read the terms and conditions carefully before claiming any offer.