1. Why is engaging in said consulting the highest-and-best use of Tesla's resources, from a profit maximization perspective? The fact that there's a precedent set with SpaceX isn't a justification, since SpaceX is another toy in Musk's toy chest, so the conflict of interest remains.
2. What rate is Twitter / SpaceX paying Tesla for said services? Are those friend-prices or market-prices? If the rate is either too high or too low, Musk could be accused of self-dealing.
3. What kind of bidding process did Twitter put out on these consulting services before settling on Tesla? If I'm one of the investors funding this Twitter purchase, I'd want to know. I wouldn't be surprised if the SEC wants to know, as well. How do Twitter investors know that they're getting the best bang for their consulting buck with engineers whose industry focus so far has been automobile-specific?
4. Lastly, what work would these engineers otherwise be doing at Tesla, and what happens to the product roadmap of those teams now that they're working with fewer resources? Musk is already notorious for promising new features are just around the corner, and failing to deliver. Missing a launch date because supply chain issues is one thing; missing it because the CEO yanked key engineers off the team to go QA his new hobby horse is something else. Such a date miss could have financial implications for Tesla, which would negatively impact Tesla investors.
Elons got a large bench of people who cheerled this thing, ask Larry Ellison for some people who can go audit this thing over multi-months.
Additionally, there are various institutional shareholders that don't operate based on their personal opinion of Musk/Tesla, but have specific legal duties to protect their investment in the company.
The SEC could still act on the issue even if it was somehow proven than not a single shareholder actually cared.
EDIT:
> should-ing on behalf of other people is not often particularly useful unless you're already powerful.
Solidarity labor strikes, where one union strikes to show support for an unrelated striking union, would be a counter-example to this. As individuals, the striking workers are not powerful, but collectively they can have an effect.