Illegal? It is the law. From what I recall, it was a gift to some senator in the late 1940s in an unrelated political bargain. Even then, it was meant as a temporary measure - even codified as such into the law. But Congress (R), in their infinite wisdom, bowed to lobbyists and made it permanent in exchange for some rather large campaign contributions. So there it is. It is literally the only industry immune from anti-trust. Not only anti-trust enforcement, but the even investigation of anti-trust.
Go figure.