In my opinion, the gambling site should have programmatically made it impossible to bet from the bonus account on the listed games and introduced the necessary limits. Otherwise, it all looks very much like a not entirely honest business, with its creators covering their actions with a user agreement.
Most of them do restrict games that are not meant to be played with your bonus balance. It's so silly that the OP lost his winnings this way...It's like a restaurant has poisonous stuff in their menu but they stated about them somewhere on the last page of their menu with 2pt font size.