It's one thing to charge for bags that are gate checked because you knew in advance you couldn't bring the bag. However, another thing that happens is you pay

It's one thing to charge for bags that are gate checked because you knew in advance you couldn't bring the bag. However, another thing that happens is you pay for (or the ticket includes) a carry on bag but because the airline has insufficient carry on space to allow all passengers to bring a carry on, they collect bags at the gate to be checked as cabin bags instead (even if you hadn't paid for a cabin bag). It's the latter that would be surprising to see airlines charge for, as it's a solution to a problem for the airline that the airline created itself by overselling the carry on space.