Legally, it is. Legally, Appoeis in the right. If people don't spend money on other platforms, why should Apple be penalized for it? They have no monopoly. They're doing everything by the letter of the law.
But having exclusive control wrt apps over a user base generating a (vast?) majority of profits in mobile is very significant (unprecedented I believe?). Vendors cannot ignore iOS so they sign on the dotted line because there really is no choice. Not servicing iOS is not really a choice, there is too much money.