Another reason why Glassdoor takes down negative reviews is that often the 1 or 2 star reviews don't follow their content guidelines [0].
- [Your review should be truthful and constitute your own personal opinion] -- Any review that contains false information that the company can easily prove is wrong can be removed.
- [Each individual is allowed one review, per employer, per year, per review type.] If the company can prove a review is a dup, it can be removed
- [We do not allow reviews that include negative comments about identifiable individuals outside of this group.] - A review saying "My coworker Alice was a real A Hole." can easily be removed
- [We realize talking about work can get emotionally charged, but we don't approve reviews that include certain profanities] -- Anything that's aggressively worded "I worked at this company, it fu* sucks" will be removed
I'd guess than 90% of Glassdoor reviews removed blatantly break their guidelines. There is probably another 10% that consist of companies trying to game the system, but it's impossible to know for sure.
[0] https://help.glassdoor.com/article/Community-Guidelines/en_U...