I saw my reviews on the interview process for IBM and couple of other companies disappear without trace. So my guess is their business model is trying to get companies to pay to remove bad reviews.
I worked for one company where everyone who left started posting bad reviews. The next thing you know people who were still there were posting wonderful reviews and getting an "official response" from the company.
I reported all of them to Glassdoor and sent them the PR email, the reviews are still up months later.
edit: just checked again and everyone left fairly truthful 1* reviews in a short period of time a few weeks after I left, I'm guessing they did another round of "please leave positive reviews" and it backfired.
Seems to have taken the opposite strategy of Yelp, which bumps one star reviews to the top and makes companies pay to hide them. Or my ex-company is already paying, it's hard to tell.
I won't go as far as to say that they remove reviews purely on request, but I've seen removals due to requests that were based on what I felt was a pretty shaky justification.