In the specific case of Glassdoor, leaving a review about an organization sometime requires a proof that you work there, e.g. receiving a pass code sent to a work email. I'd say that this is reasonable, and makes gaming the reviews much harder.
replyCan't the bots just sign up for accounts?
replyYes they can. But then you see which account as took part of the DDOS scraping, and delete all accounts that match the pattern and site gets back under control.
replyFundamentally it's all a game of whack-a-mole for admins unless some kind of microtransaction system is invented. Then a DDOS scraping event is just extra revenue.