That's not what I say, that's what The Constitution says. You answer the question: Who makes the license plates?
What gives the state of Texas the right to walk all over the First Amendment and what other speech can they suppress? The state opened the door when they decided to offer vanity plates and set the standards for qualifying for an individualized design. If the SCV met those standards then the state should not be able to deny them their plate just because they don't like the message or the messenger.