Then why can't WhatsApp just send SMS messages? Apple's Messages app already supports that. If you're not looking for feature support, then it's already done. If you want feature support, then we end up with some sort of bizarre incompatibility matrix, listing "if you're in program X, talking to someone on program Y, then A, B, and D work, but C, and E through K don't work". Great experience that'll be. Government regulations is not the way to get good APIs to happen.It's for the Apps to ask for interoperability. It means, if WhatsApp asks Apple to allow interoperability with iMessage, then Apple must allow it. How Apple would do it probably is creating a few general-purpose APIs that will allow access to any messaging platform. They have to use the APIs to create interoperability. It does not mean every feature in iMessage will work in WhatsApp. It just means that the APIs will allow a person on WhatsApp to communicate with a person on iMessage directly.
It'll also mean entries showing up in your messages essentially saying, "Message from a source that WhatsApp purports is Jane Doe". Apple won't want to just trust whatever name WhatsApp (or whatever service) gives for the user on the other end, and that user won't have an AppleID (that Apple can trust, coming from WhatsApp - what's to keep WhatsApp/etc from saying "oh, yeah, the AppleID is tcook@apple.com". So, your native Messages app can't trust the identities beyond "WhatsApp says this is 'foo'", and the Messages app only works with AppleIDs and phone numbers. I haven't heard anyone talk yet of mandatory phonebook compatibility between devices.
This also seems like a great way for scammers to send spam messages, from some service that's easy to get accounts on, through these new gateways, onto other services, with limited potential for the receiving service to combat the spam.