Because you staked £20 for that buy.. that’s where you breached the terms. Yeah it might have been 20p per single spin, but you technically staked £20 to buy it. That being said though, if you can manage to get the max bet hired to £10/£20 like some streamers have, they will accept bonus buys that way. If you’re unsure of something in future, always ask chat support, screenshot what they said and then if they’re wrong and try to confiscate funds after you try withdrawing, you’ve got proof of what they said.