Vlad, except for very few countries which do not even allow inert grenades (mostly not allowing other inert ordnance too like France and Denmark, the Dutch need a license I think, yes for inert stuff!) it is ok as long as they are inert. I never heard of a country that allows life grenades in private hands(except for smoke).
Of course all will end the same day some maniac will take kids hostage in a school with such an (inert) item. Then our brainless politicians will even ban photos of such for publicity reasons and panicking media which will earn even more money with dramatically made up headlines (for exactly that reason) causing clueless public to suddenly have an opinion on something they know a jack about.
Collectors of grenades are (still) quite a big community over here.