Too much on my plate

A federal appeals court ruled that the State of Arizona, and other states, must allow license plates with the message Choose Life.

The appeals court said the message is private speech by both the anti-abortion group that sponsored the message and the owner of the vehicle.

I'm not a legal expert (though I've played on on TV) and I'm sure this argument was (hopefully) brought up, but if the numbers and letters on the plate are assigned/approved by the government, how is that private speech? How is it private speech if the state issues the plates, requires them to be posted on vehicles and can seize them from the vehicles' owners?

Would a point of view 180 degrees ( like Choose Choice) be allowed? How about the literal opposite: Choose Death?

License plate trivia: when California first required registration of vehicles, the State issued the number. Each motorist had to then create their own "plate".