Because if they were to fire him, then they'd basically have to fire them like a week or 2 before his vesting cliff, with an establish history of an attempted negotiation.
Firing someone a week before they vest, in order to claw back the shares, because the other person refused your offer, is likely illegal, and not "good faith".
If he is already in a situation where he has nothing to lose, you may as well force the other person to engage in the possibly illegal action against you.
Or, at the very least, get them to make the illegal threat in a way that you can document it.
Even the bluff, or threat of legal action, against a questionable practice, is a huge problem for a company, that most founders do not want to have to deal with. And he doesn't have anything to lose anyway...
That doesn't mean you have to sue them right now. Even the threat of mentioning that this might be illegal, and that you are at least considering legal action, could force the other person to give in, and not do the possibly illegal thing.
Or even beyond that, if there is a documented evidence of these illegal actions, what you can do is simply not sue now, and only sue later on, if the company is actually worth something in the future, and it makes sense to do so. All upside, and no downside. Don't pay the court costs, unless there is something to gain.
