Nothing prevents private citizens from adding whatever political statements they want via bumper stickers, signs, etc. I certainly don’t agree with the pro-death crowd, but we can’t banish political opinions we disagree with. If we allow pro-life statements on government issued plates, we have to allow the alternate. How would you feel if the situation was reversed?
What do I care what people choose to put on their license plate? I'm not one to seek to silence anyone else. People tend to know what goes outside the bounds and censor themselves just fine (typically through ridicule and humiliation).
Frankly, though, it would be refreshing for the Left to take off their masks and proudly support their own agenda. Long live fascism, government dependency and the culture of death! I can see the license plates now... "Don't choose life, Choose Death!", "Save Gaia... Die." The fact that they don't put their views out there front and center but rather just want to silence me is the crux of the problem.