It is stealing and copyright infringement. The thing is though (i heard) it is in fact very hard to catch and sue these people due to a few reasons. The biggest one being where the private servers are being hosted. I may be wrong, but i assume (judging by all the counterfeit things in these countries) that the copyright laws are not very strict if they exist at all. China is one of the biggest. That being said, i have no idea where the private servers are being hosted, but i would assume none are in NA or EU and if they are i doubt they will be up long. Yet another reason to NOT play on them. They can simply close at any time and say a big F.U anytime they wish.