This is completely normal practise for bonuses at nearly ALL MG casinos - so I really don't understand what you are complaining about.
You can't expect casinos to give you free bonus money and let you just cash it out straight away - they would go bust in days!
KK
The OP might be saying that the bonus balance can't even be PLAYED, rather than just not withdrawn, until WR is completed.
This might be down to the site actually blocking access to prohibited games, which might make it appear as though the bonus balance is unplayable.
Their other bonus terms, such as banning certain slots like Scrooge, would not be necessary were the bonus balance to be unplayable. These terms cover situations where a playable bonus balance can be used for advantage play.
The terms for no deposit free spins clearly cover the bonus balance being playable, as they limit play to the slot the free spins were given on until WR has been completed.
It's not just a Microgaming casino, but a browser based platform hosting games from multiple suppliers.
There is also a VERY nasty term. If a progressive is won on any game supplied through one of their providers (BetSoft games), there is a 20% levy charged on the progressive win before the player receives it. This charge is claimed to go back to BetSoft to re seed the game.