Doesn't matter where they claim this fee is legitimate, if the person refuses to pay (which I would), there's no way this would be upheld in civil court.
I'm not sure that I believe this. I can believe that there are some places out there that think that they can get away with enjoining you against posting negative reviews (until the law eventually settles down against that, as I'm sure that it will), but I find it hard to believe that there are many places that think that they can fine you for the actions of other people. Can you give examples?
EDIT: As baddox (https://news.ycombinator.com/item?id=8131575) politely points out, I worded this badly. I want to say something like "the actions of other people over which you have no reasonable expectation of control" (for example: you can reasonably be expected to prevent guests in your room from damaging the furniture, but you cannot reasonably be expected to monitor the Internet usage of all your guests after they leave to make sure that they do not post a negative review).
There are plenty of examples of that, even in the context of hotel booking. The person who pays for a hotel will be fined if non-paying guest damage hotel furniture, for example.
I'm not a lawyer so maybe I'm missing something obvious. I would just like some substantiation. It seems that many people have the habit of automatically declaring something they don't like to "must be illegal" simply because they strongly dislike it. That's now how law works, and that's important to understand because many people seriously underestimate the powers that the legislature holds.
I didn't claim that it was (il)legal or (un)reasonable, only that I wasn't convinced that language of this sort (where you are being held responsible for the actions of people when they are not under your immediate control) was common in contracts.