You have it exactly right. Companies are protecting themselves from more than just legal action - Glassdoor reviews have a huge influence on how candidates view your company (especially true for startups).
I once left a bad review on Glassdoor after a truly awful interview experience for a startup. The next day they called me up and threatened legal action.
Edit: Just for context on the interview—they had me do a take home assignment in Django, which I had never worked in. Was desperate, so I learned Django and completed it in a week. Went onsite and their whole office was a one tiny, dated swelteringly hot room. The interview was mostly around a bunch of basic syntax stuff, and had me implement merge sort. Didn’t hear from them at all for a month so I wrote on Glassdoor about it.
A day after I wrote the review, they tried to email me and offer me the job. I was pretty desperate, but not THAT desperate knowing they had only done so because of the review. So I replied, saying I wasn’t interested. The day after that is when the CEO called me to threaten me. They probably couldn’t have done much to me, but still nothing for me to gain from the situation, so I took it down and moved on with my life.
