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.
