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As far as I know it's illegal to manipulate the market like he did.
He bought BTC, tweeted about it, price rose, he sold, tweeted that he doesn't like it anymore, price dropped.
Cryptocurrency market is not regulated like stocks so I doubt he'll be indicted for tweeting about what Tesla did. The company bought BTC and we found out about that when they filed a report. The company also announced accepting BTC as a payment method. Lastly, they decided to suspend it and willing to resume after bla bla bla.
Those who got caught by the dump may feel cheated but it's also their fault for speculating and buying from someone's tweet.