I researched this a lot because I noticed companies I had intimate knowledge of had any sub-4 star review disappear.

I believe you are correct that Glassdoor wont explicitly take “cash” to remove a bad review. However they do have internal costs if you threaten them legally, which can compel a removal.

I detailed more of the practices in another comment further down; https://news.ycombinator.com/item?id=24793383