That is really shitty behaviour, and bizarre. Glassdoor is only feasible if users can have trust that their feedback is anonymous. I really hope this has not harmed anyone, I'd imagine the lower rated companies are most likely to retaliate.
It may make users virtually unemployable if HR starts using Glassdoor as a database of publicly critical people. Given the choice between engineer 1 who never posted anything bad against a company, and engineer 2 who posted critical comments, who would you hire?
Although it probably gets you further than the opposite extreme where everyone disagrees all the time.
Personally having such info (critical reviews on previous employers) for a candidate, would be close to already having their answer for "Tell me about a time you had a challenge/problem/conflict at work" type of interview questions.