Before the trial happens, it could really go either way, even if the defendant is obviously in the right - there could be some procedural slip up that causes them to lose anyway.
However, a lawyer isn't going to assume that they will make some slip up, so if it is obviously in the defendant's favour they will work pro bono. There is still some risk for them, because if they lose they don't get paid, but they're confident they'll win.
Edit: wrote the reply thinking this was a conversation about awarding costs to the defendant, that was a different thread. The first paragraph remains unchanged though.
I wish Lemmy showed you more of the context than just the last reply.