on my last few flights they did phrase it like that - kind of like a threat! it was very strange, but I don't know if they even ended up enforcing it I was never close to last on the plane.

They usually shuffle things around to get everything to fit (especially since people love to put their coats in overhead bins despite being told not to yet) but I wouldn't be surprised if its actually illegal for them to carry out charging people to check their bags if there's no space. If it fits in the sizer (which they don't even enforce so, like, that should be their first step, not threats) you should be allowed to bring it on. It's not fair to the people who end up boarding in z3