Probably unpopular opinion : but how are things like these that Musk does w.r.t publicly held shares legal?

Strictly speaking, it’s not. He’s violated securities laws multiple times but seems to feel he’s immune to prosecution. Consider the recent “financing secured” tweet.[1]

[1]: https://www.theguardian.com/technology/2019/apr/26/tesla-elo...

He has violated them once and got punished for that. And then there are 100s of claims by uneducated people that he 'constantly' violates the law.

No-one ever proved he didn’t. Look at the share price now - it would have been a damn good deal!

It drives money into the system, popularizes the markets, and also because it's probably hard to make a case in the current state of things. This is America, big corps get preference and grace. I'm not saying it's right, I'm just saying that's my assumption of how it is.

The tweet doesn’t have material information about the company. Also, holding a poll about selling shares would probably be upheld as protected speech under the first amendment.