It probably isn't, but it's in that annoyingly small fee zone where it costs more to take them to court than it does to just accept the fine. I think in this case, they'll probably get the money back from their CC company though.
Under the UK Consumer Credit Act, when a credit card charge of £100 (or more) is made, the credit card company becomes 'jointly and severally liable'. A charge for a bad review also falls foul of UK Fair Trading legislation which means the couple can pursue the credit card company for the unfair trade breach and reclaim their money that way. Credit card companies don't argue in these cases and refund pretty damn fast (from experience) and just charge-back the business (so they lose the £100 and have additional charge-back fees of around 25% added on top).
If the hotel charged £99.99 instead, it would be a very different story and would need the Trading Standards people at the local municipal council to take action. (They already stated they will be regardless of the refund since these are unfair trade practices.) A refund would happen but would take longer and may well need a local county court action.
https://www.gov.uk/make-court-claim-for-money/overview
They could get free legal advice from Citizen's Advice. http://www.adviceguide.org.uk/wales/law_w/law_legal_system_e...
But just reclaiming the money via credit card companies is probably easier.
I'm curious about why the people leaving one and two star reviews are not also sending reports to trading standards / environmental health. If the public areas are dirty I dread to think what the kitchens are like.
These kind of petty fines really annoy me, far more than a lot of major crimes, because people know exactly what they're doing when they pick a low amount - and they expect to get away with it. You see them all over the place, eg. change your ISP and they'll fine you £50 for not returning a £10 junk router that you have to return, instead of them picking it up.
The money is in an annoying zone where you are put off filing a claim because of the potential cost, not just in money but in time as well. You might still need to go to court over your small claim, which will take up a day and a lot of hassle, etc.
It's one of the reasons that parking tickets are only £30. You could contest them, but it's costing more in terms of your time than you'll recover monetarily, even if you prove the ticket invalid. Likewise with automated speeding tickets from cameras - you could contest it, but it's just too difficult for most people to be worth the risk. Whereas in countries like Switzerland or Finland, where they can have huge fines, I imagine they have a lot more court cases...