selling babies on the black market is also not legal but been around for decades.

Limewire is not illegal. Just like cars are not illegal. It's what one does with it that can be illegal. I can own a car, but there are things I can and cannot do with it. If I kill someone with it, I've done something illegal. Even if my car is paid for, inspected and registered.

Limewire is just a tool, what you do with that tool is what can be illegal.

However, at the same time, there's plenty of legal things you CAN use it for and those are the main reasons (among being spread out and not centrally located) that keeps the program itself legal.