It is a legal rule that the company decided to impose in it's corporate charter, they can amend it to be whatever they want. It's arbitrary and not a fundamental part of US/State corporate law (like say race discrimination or workers comp). If they wanted to put a law in that you need to spin in circles and tap dance or else your stock automatically gets reclaimed they could create a clause in their charter for that too. This is what people need to understand: Rules change because people wake up and push for them to be changed.
Lawyers have created all sorts of bullshit to protect the employer and that has become engrained as "best practice"
No, this is a provision in the agreement that's driven by the tax code and IRS regulations. It is required if you want the option to be an incentive stock option. It's not arbitrary.
You're right that you are free to to negotiate a different exercise window prior to accepting your stock grant (and a lot of more progressive companies are offering this). You just can't, under current tax law, get the sometimes beneficial ISO treatment with a longer window.
You are right -- it's not as black and white as I described.
That being said -- It seems like with some "legal engineering" it can certainly be ameliorated (as was demonstrated by Pinterest). My main point is that if people push for their rights then it will incentivize companies to do the legal engineering that is necessary.
Lawyers have created all sorts of bullshit to protect the employer and that has become engrained as "best practice"