There are at least 4 reasons they are "banning this" that I can pick out right away and all of them are pretty solid/justified in the context of what's going on outside the WWE and the past.
3 more guys have just launched lawsuits based on concussion, that's 3 or 4 seperate ones all going on at the moment. They all are alleging WWE doesn't "take care" of it's guys when it comes to concussion, which is also a feature of their doctor suing CM Punk...
So they have to be seen to be reducing risks and that means moves that even LOOK like they could cause a concussion will be riskier and reduced. The Curb Stomp is arguably a "safe move" but it has the appearance of one that is not, and how they often use it...like the cinderblocks with Ambrose, could easily be used as ammo in those lawsuits.
The second reason is a little sadder but also very pertinent.
There was a case several years ago, many of you won't even remember the name Lionel Tate but he did as much if not more damage to the WWE/F as Chris Benoit ever did. This was a "big hoss" of a kid who used a Rock Bottom to kill his toddler sister. It went to trial and it was very messy for WWE. Remember all those "Don't Try This..." rather than "Don't Try this at home?" That was why they got more serious.
The curb stomp is a move that kids could already be "using" on the playground etc...on things like cinderblocks or suitcases. When Seth does the move, the opponent knows how to take it, even on an object and it's clear he never actually makes more than a brushing contact. A kid trying that on a cinderblock, pavement or even a sofa cushion and they get it wrong and it's a potentially fatal situation. So WWE is taking a morally sensible position and removing it now while Rollins can still get over. He has other things he hasn't used so far so he'll be fine but if they stop one kid getting a broken neck or crushed skull by banning the move it's worth it... with the recent death in Mexico as well, that no doubt has played a part in the decision, anything where a kick to the head or neck is in play is under threat. Dropkicks won't be as high any more for example.
Sad but necessary until the formalities and any lawsuits down south are completed. Rey could still yet be sued, if it was WWE it happened in, they would be without question.
Again younger members of the forum might not remember the name Chuck Austin. This was a guy who botched taking Marty Jannetty's "Rocker Dropper" finisher and was crippled for life. Even in court he admitted he hadn't got the move right, but WWE still lost and it cos them $26m in damages, that was in the mid 1990's... today that would be tripled and probably directly linked to their worth... so we come full circle to the first reason. WWE doesn't want ANY more of those and the curb stomp typle move is a Rocker Dropper, just with the sole of the boot not the leg.