I'm going to explain just one more time:
A Bonus Buy, when there's an active bonus, is not allowed (it's a standard rule at every casino) - the system allowed it because the platform itself may not have been fully equipped to disallow it, or that the casino system was lacking (platforms usually do have setbacks and lackings).
Your approval was approved probably prematurely, until they realised later it was won from a Bonus Buy.
They returned your 200 EUR because that amount of bet was not allowed (which the system failed to disallow) and shouldn't be deducted from your balance in the first place, hence the refund. The 12000 EUR shouldn't have been won from a 200 EUR bet, hence the confiscation.
The casino is being honest by refunding back your 200 EUR. That's not picking and choosing as they like either.