I would say it's not so much about 'allowing' pax to bring it, but having to make the call between wasting time arguing with them at the top of the slide to chuck the bag away or to let them down with it and keep them going out the door...
In some parts of the world you will be lucky to be standing holding on the assist handle without being shoved out by the pax and their oversized bags!
If it means I can get my arse out quicker, I'm not going to waste more than a second yelling 'Throw it aside!' before deciding if it'll be quicker to just let them get off with it....
Don't be too quick to condemn the CC....