> All that means is that controlling shareholders can't use the company as a piggy bank and raid it to fund their other ventures

Yes, you're getting it now.

> It doesn't mean the business has to be "sustainable" or whatever. In fact, it's perfectly legal for the board to sell to a "vulture" PE firm that will sell the business off for parts, as long as the sale price is good enough.

As discussed elsewhere in this thread - the sale itself is required to maximally benefit the shareholders.