Right now a Verizon iPhone customer can't securely send a message to a Verizon Android customer without forcing their customer to download a third-party app and agree to its terms, and the carrier has no recourse to change that on their iOS devices they are selling to their customers.
Vaguely reminds me a little bit of the web browser antitrust days. Regulators will catch up soon. At least Apple is planning to adopt RCS finally, even though they missed their chance to be part of the formation of the standard.
Edit: Or Apple could release iMessage for Android and for web.