I've read from blog posts and even from conversations that companies require acquihires, particularly non-founder employees, to go through an interview process. It may be shortened, but they still often have to do the typical algorithmic interviews.

I'll see if I can find one of the blog posts I've read about the process.

Note: to be clear, I'm describing the process for acquihires, not founders just wanting to sell their company and walk away with some cash (which actually seems to be somewhat unusual).

Edit: this was discussed in a similar thread on HN just last year: https://news.ycombinator.com/item?id=18943500

Is that a uniquely American thing? I'm 100% certain that even if the company I work for was acquired, my current contract would still apply. So I would need to be given at least 3 months notice, and couldn't be discharged without a good reason - failing some internal interview wouldn't be such a reason.

Perhaps. Most places in the US an employment contract can be terminated at will for almost any reason or without any specific reason.