Why is it legally easy for them to do this, but in other high profile examples it was difficult for companies to dilute a now-unwanted partner? For example, Gates and Ballmer pushing out Allen, or Zuckerberg pushing out Saverin.
so, it's a lot easier to do this kind of thing when there's a downround and the alternative to these terms is the company shutting down - basically, the new folks diluting old folks can say this is ultimately in the best interest of the company because the alternative is a shutdown.
in the context of MS or FB, both companies were on the up and increasing in value, so harder for the big stakeholders to say pushing out another SH was just about "serving the best interest of the company" with a straight face.
IIRC Zuckerberg and Thiel created a new entity and gave Saverin a difference class of shares/equity then diluted that class. They must have known they'd never get away with it, it was likely just a move to be able to fire him quickly and just cut a check at a later date.