If they don't violate NDAs and the allegations aren't true, why shouldn't they be able to?
Finally, if Glassdoor is anonymous, why would they have any records of anyone who posts a review? If you want to claim some content has an anonymous author, you, the company hosting the content, cannot have any ability to identify the author. Otherwise all this "anonymous" info is a hack, malicious employee, or lawsuit away from no longer being anonymous.