Hello, although it seems like a very small amount to bet, I also found it in their Terms and Conditions. 
To be honest, I've hardly ever come across this being the maximum bet when playing with a bonus. I'm very sorry to hear about this situation, but it's always best to look up exactly this kind of thing when you play with a bonus, because it's a nightmare to find out this in the end.
I hope you won't come across something like this again.
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