Well, actually, that's because it very much is. Just Disneyfied/Enchantedified.
So that's make Fables a rip-off of The 10th Kingdom.
No, there's a big enough difference between those two for them to be different. The 10th Kingdom wasn't about The Big Bad Evil excommunicating the faerie tale creatures to the real world, where they are then forced to live as humans in a small community while trying to find a way to defeat The Big Bad Evil and return home. Once Upon a Time, however, is. The only real difference between Once Upon a Time and Fables is that there isn't a war raging back in their homeland (instead, Once Upon a Time just has a bitchy queen serving the same role, complete with an army), and they're brainwashed suckers rather than homeless refugees. That's about it.
Willingham himself said that his idea is not original. So what I don't understand is why all the fans of the comic gets upset and say things like that.
Being inspired by something is one thing. To rip it off is something else entirely. Once Upon a Time is a cheap knockoff of Fables with Disney characters and a less interesting Big Bad Evil who are brainwashed into living as normal humans in one small community rather than simply pretending to be normal humans in one small community.
Neither of those are anything like the 10th Kingdom aside from the fact that the faerie tale creatures are real. Likewise, Grimm isn't a rip off of Fables or the 10th Kingdom despite also involving faerie tales. Faeries tales alone don't make them rip offs. Stealing plot ideas and major concepts is.