Is there any proof this is happening other than this post? I hope so otherwise internet vigilante justice is destroying a business and people's livelihoods.

Edit:

I took a look at their site and apparently they only enforce this policy for weddings:

"Also, please note that we only request this of wedding parties and for the reasons explained above."

The reasons seem to be that as guests are not booking the hotel themselves (presumably the bride/groom has done that for them) it may not be to their tastes and they feel it's unfair to leave a negative review of something you never would have booked in the first place because it's not to your taste.

I can kind of understand this reasoning, although it's not great.