You can’t cash out the bonuses on Tropicana, just the winnings from bonus bets. This type of bonus is called a sticky bonus, and I think Trop/Virgin are the only sites that use them.
Sticky bonuses actually aren’t that bad. What I usually do when I get one, is wager the whole thing on one hand of blackjack. If I win, great, I can withdraw the winnings immediately. If I lose, so what, it was a freeroll anyway.
Sometimes I like sticky bonuses better than traditional ones, because I don’t have to wager them 10 or 20 times before they clear.