What's wrong with encouraging employees to post reviews? When you sign up as an employer, glasdoor gives you a sample email to send that asks for honest reviews.
Let me rephrase that last sentence: GlassDoor was brought up literally at every monthly update meeting.
But back on target, I'm torn: On the one hand I understand the need to actively recruit and attract talent to join your workforce and I definitely understand GlassDoor is something used by many people to benchmark their expectations on working environment, management and peers. I get that completely.
Where I break off though is when you have to ask your employees to write a review (even if you're adding the qualifier "Be honest, we wont penalize you") instead of creating an internal feedback environment that can go into recruitment, retention and employee/manager review platforms.
I would say there's already a power imbalance inherent to the workplace, especially if you're in one of those "at-will" states. So putting in front of your employees "Hey, can you go write a review of working here on this site?" understandably makes some people shift in their seats.
Unless you think people fear being ID'd... but that's a problem with the whole model.
Anyway, I'd imagine many glassdoor reviews were generated this way. If employers didn't ask, it would be mostly bad reviews from ex employees. Few others would bother.