The resignations feel less less like a result of a mismatch and more like result of a slap in the face from mismanagement.
I'm a firm believer that you can help toxic people become normal (again). But it takes a lot of time and effort that you can't spend otherwise, and you might alienate others over it who don't want to suffer under the (maybe reforming) toxic person. The question is whether you want to help someone, or build products. Basecamp seems to have opted for not turning themselves into a SuperPAC/self-help group.
I'm not sure about that. In my experience, it's about priorities, and some people find e.g. political activism more important than building products. When they have to decide between "more activism" or "more products", they value activism higher.
I don't think it's a junior/senior thing, and not everybody will end up where they feel like they + the company have "grown apart". In this case, from afar, it looks like most in the company (including the founders) where pretty political, but some employees got more radicalized and the founders (who essentially set the company direction) did not.
It's a kind of schism, but it's best for everyone involved to let it happen. I find that kind of generous buy-out a very good way to do it. Some have suggested that it's some kind of demonstration of power, but I don't see that: in a power play, they would've fired the problematic employees (and having worked with activist employees, I'm confident that everyone knew who they were), not given them a large bonus for leaving. They've extended the bonus to everyone (which is great as well, you're not rewarding the bad apples, to so speak), and I wouldn't be surprised if some took the offer purely because of the money.
Firing them is a good way to tip them over it.