Free speech?

For the state not to create a particular license plate (eg. with the Confederate flag) does not keep anybody from putting their own bumper stickers showing the Confederate flag on their car, if they are so jerky. If someone wants the state to issue a tag honoring rhubarb, would the state be obligated to do so? The fact that most of the Republican appointed justices could not see this shows the low level of rationality of the kind of people nominated by Republican presidents.