The issue isn't about signing the petition, but that you have to be a registered voter to sign it. So, as the legal pundits say, it's paying at least some people to register in a round about way. And that is most likely the intention of the reward and has little to do with the petition and is basically illegal but a loop-hole in it's manipulation. That's my take anyway.
People can say poor Elon is being picked on, but it's just the way the law is. I'm sure his lawyers get paid more than government prosecutors, so he's probably in the right.