How are you going to make a case for tortious interference when the would be interferee is profiting by using the interferer’s resources without payment?
From beepers website, there’s no use of apples servers when iMessages are sent from a beeper user to a beeper user. Rather, they only pass through Apple when sent to an iPhone user and in that case it’s the iPhone user that’s utilizing apples resources. And in that case there’s an Apple device owner, who is paid for the right to use iMessage servers.