Why would this be illegal?
Consider this scenario: you have an iMessage group chat of 3 people, and one of the three people has a plan like PayGo[1] and an extremely limited monthly SMS quota. What happens when one of the other two participants suddenly counts against that quota?
There are options, of course, but for consumers as a whole those options are hostile and inelegant - are you gonna pop up a warning saying that messaging these people might now cost you money?
I guess you could make an argument for turning iMessage off by default, but that also seems like a net negative for consumers as you're back to giving phone companies the opportunity to charge exorbitant rates for things like MMS. Then you'd just push everyone to yet another third-party messenger that could be acquired, aggressively monetized, or shut down at any time.
Just dealt with this with a sibling who switched to android. How is this legal? It’s quite ridiculous.
I wish everyone in the US just used WhatsApp or another third party app like it seems they do in every other country. Instead, not having an iPhone makes you a pariah because of the green messages.