Not being able to rely on RCS from rooted phones makes sense security-wise. You can’t trust what’s attached to that message. It could be a code injection hack.

It makes no sense at all security-wise. It is always a bad idea to rely on the security of a client that you don’t own or control.

Your statement makes the assumptions that Google can detect every rooted device and there’s no possible other way to send RCS messages. Neither are true.