I don't think you need a tying argument at all. This is a deceptive business practice and harms apple's customers moreso than non-apple customers. Apple users who expect their text messages to be secure because of how imessage has been advertised to them send insecure messages in the same application based on a difference between people they might not even know about upon receiving their phone number (or that may change if that person switches phones), and the only argument I've seen people make is that Apple wants this to be some "exclusive" service, and this Beeper case has made it clear that they're willing to try to enforce this in the face of someone else interoperating with their protocol
Our default position on businesses, especially large ones, should not be that any agreement or restriction they want to hold their customers to is legal, but even setting that aside, this is deceptive advertising that harms their own customers
1. Apple makes it clear that this indicates whether someone has an iPhone. This does not make clear to non-technical users that this indicates that you are sending a different type of less-secure message to these users.
2. Even if Apple made this clear, it still creates a pretty insecure environment in practice. If you don't know ahead of time what phone someone you're talking to has, you must receive a message from someone before you can know whether messages you have sent them are secure, in the same chat application you use to send secure messages
Our default position on businesses, especially large ones, should not be that any agreement or restriction they want to hold their customers to is legal, but even setting that aside, this is deceptive advertising that harms their own customers