They can't sue him because he refused to sign the NDA they were trying to get him to agree to, incurring extra cost and hassle just to preserve his ability to talk about what happened. In fact, fans on the Tesla forum have been taking this as proof that he's really just out to destroy Tesla, arguing that if he wasn't just an anti-Tesla troll he'd have taken the 50% discount on repairs and associated gag agreement and that the fact he was willing to pay an extra grand just to be able to discuss it proved this.
This is also why this sentence from Tesla is incredibly dishonest:
"It is deeply ironic that the only customer who apparently believes that this document prevents him from talking to NHTSA is also the same one who talked to NHTSA. If our agreement was meant to prevent that, it obviously wasn’t very good."
Of course it didn't prevent him from going to the NHTSA, because he refused to sign it in order to preserve his ability to talk about it, paying an extra $1,500 to Tesla for the privilege. Anyone who did sign it in the belief that it prevented them from going to the NHTSA is also forbidden from talking about that by the NDA, so we're obviously not going to hear anything from them. Really sleazy, and seemingly par for the course for Tesla PR these days.