Did they force you to gate check your bag, and then charged you a fee for it? That's what seems to be claimed here. I'm also pretty sure it'd be a violation of the carrier contract.

If it is within your allowence, but there "isnt enough space in the cabin" there will be no fee. If you turn up with 3 bags, bags that dont fit, or overweight bags they can and will charge you as a checked bag + a handling fee for the inconvenience.

You fly jets long enough, something like this happens.