Apparently a company called VirnetX is seeking damages from Apple over iMessage and FaceTime, which some people (in my previous discussion among others) point out as their excuse not to develop cross-platform iMessage as they do cross-platform Apple Music. I don't have any evidence of Apple themselves claiming this as the excuse.
https://9to5mac.com/2016/05/26/patent-troll-facetime-imessag...