It's a matter of perspective, really. From Wilder's perspective Breland knew full well in advance never to throw the towel yet he did it anyway (whether his intentions were good or not, and they were, is beside the point). Wilder has changed his mind about Breland staying on the team because he's naturally concerned it could happen again in the future.
From our perspective him canning Breland, who had been with him since day one as a pro, looks like a dick move, but we're also not part of Team Wilder. If somebody on my team couldn't be trusted not to throw the towel if I'm in trouble then I'd axe them too. What other option would you have aside from benching Breland for the most important fights of your career? And, if you're going to do that, then why pay him as a co-trainer at all if they won't be there to work your biggest fights? You'd have no other choice.