Is this an US thing?

I went through three acquisitions and never had any interview, besides the set of meetings for each of us to decide to go along with the acquisition or get a severance package.

I think it all depends on the requirements of the company doing the acquisition. I've gone through several myself and did not have to re-interview however I have heard of a employees claiming Google required them to re-interview which has got to be nerve racking considering Google's heavy focus on avoiding false positives a lot of good people don't pass them.