Many websites would also like to claim that you're not authorized to access them with an ad-blocker, and spammers would like to claim you're not authorized to use e-mail with spam filtering, and malware authors would like to claim you're not authorized to use antimalware software since all those countermeasures encroach on their respective business models.
Okay, but the concept of unauthorised access to a computer system isn't a new one, Kevin Mitnick and co were all convicted of it in the 90s. Maybe there was an argument before Apple released a statement, but it's pretty difficult to make one now.
A line has to clearly be drawn somewhere, otherwise as per my previous comment you could also consider that ad/malware blocking is unauthorized access to websites/malware developers' servers (since you download the ads/malware but then your security solution blocks them).
The typical cases of unauthorized access are quite different in the sense that a private system is being accessed and private data may be exposed. It doesn't really apply to iMessage though - alternative clients don't exceed their authorization (the use the same auth flow as a real device and are granted the exact same privileges) and no private data is being disclosed (just like a real device, the user only has access to their own Apple IDs/phone number's messages).
The only argument that could be made is that allowing non-iOS devices access effectively freeloads off Apple's server resources, but Beeper did offer to support payment of a reasonable fee and yet Apple never took them up on this offer nor ever raised this issue, which confirms the (expected) fact that server resource usage (which is minimal btw) is the least of their worries.
