> The onus is on the participants to see if their country will allow them legally.
You typically can't shift liability that way. In particular, Amazon can't. If there's a problem and someone has the choice of going after Amazon or some local little guy, Amazon gets it, especially if said someone is said little guy.
Plus there will be arguments like "if you just changed {rule}, everything would be okay", and different groups from the same country would name different rules.
FWIW, this is an example of the constant yet mostly invisible drag of "consumer protection".