Yeah but they usually don't do it with the express purpose of screwing over an investor - and when they do, there are consequences (like FB).
You're not making a case for this being fraud. Why would this be fraud rather than a sharp-elbow business tactic? "Fraud" has a concrete meaning; for this to be a fraud, one would need to be able to identify precisely of what Musk has been defrauded.
The poison pill clause is part of the shareholder contract, often specified in the articles of incorporation.
Oh, if they execute the poison pill there will be consequences, just not legal ones.
Trust is a big part of stock value, if you don't trust the board is time to sell...
The poison pill is fighting off a hostile takeover, by someone who has offered a first-and-final-offer. It would be screwing the other investors to have zero negotiations for a better deal.