They're very portable, so you can carry them around with you in case you come up with an idea of something to write on your Tesla while you're out and about. They stick to the surface quite well and don't wash off in the rain.
(Please try not to succumb to the temptation to write "F**k off Nazi Scum" or similar on somebody else's though like other people are suggesting. That might encourage people not to buy Teslas, which is a shame, because they're great American cars.)
A car that was bought and paid for years ago is not supporting someone who had only really gone off the rails in recent years. Drive it or not, Tesla has already been paid. I guess you could argue parts or charging might continue to support Tesla, but that's marginal. Most people charge at home, and parts often come from wrecked cars as well.