You and I live in liberal democracies. We have unlimited freedom except as stated under the law. If you want an english translation of that: Its up to the government to make clear the activities that are illegal. That's very different to ignorance of the law though, which is still no excuse. Eg. If its stated in law that you cannot destroy public property you can't claim you didn't know about it. Leaving the destruction of public property thing aside, your question is about that specific sign. If it says no skateboarding or rollerblading or whatever and you want to ski on it then ceratus paribus there's nothing illegal about that. They can try to charge you but if you have a photo of the sign to show the cops trying to charge you then they'll realise the technicality. Just don't be a dick to them because then they'll get you for something else like littering or disturbing the peace, which they wouldn't have done otherwise. The logical outcome if you get caught is that they'll say don't do it again, they won't charge you, but within 2 weeks there will be signs there saying no skiing or something like "no action sports that make use of handrails" to help them cover all angles. Hope this helped.
Having been to court many times fighting fines for various things:
1) If a cop wants to cite you he won't care what you are riding.
2) If you fight the fine the judge has a lot of leeway to 'judge' you. IMO you will not get off on a ski vs. skate technicality. Judges are bigger pricks than cops and basically just do what they want. They judge your attitude in court just as much as your legal case, especially for small things.
3) If you fight the fine you may pay $55 but the city will spend over $100 fighting you.
4) There are no certainties in the legal system, there is always at least a small chance you get a cool cop/prosecutor/judge that lets you off.
4) Hit that shit anyways cause it looks sick and you should listen to more N.W.A.
i just might be your favorite skier's favorite skier
If it says no skating then you should be good. Govevrnment cant do anything to you in ex post factum laws. If they get mad at you, they cant legally change the law of the fine to apply to skiing so they can punish you. But they can make a law that will apply to any future offenses.