As I know, in many countries, what you write in your "rules" can't contradict the country laws. Said in another way: laws invalidate clauses in contracts that are more restrictive in regards to protecting rights. I'm not sure either, but I can guess that this kind of laws of liability for cancelation of events, etc., are most probably uniform across the EU.
I don't think it matters. IETF LLC is based in Fremont, CA and all events organized by it regardless of event location are subject to US law.
The fact that IETF is held in any place other then US does not mean that when you sign up to attend you are subject to your local country or region law.
Thx,
R.
