By signing a EULA, you are accepting a limitation of your freedom of speech and (along with other factors - e.g. money) gaining some compromise instead (a car). That's not First Amendment territory in the same way that moderation of an online forum isn't.
The First Amendment prevents laws being made, not contracts being signed. It means it's not illegal, not that it isn't a breach of contract.
(IANAL - Obligatory XKCD https://xkcd.com/1357/)