I do not see a problam having tags that occur only in one part of the world.
I think that is impractical. These are integrated into the rest of publicly accessible space much more than ahtletic stadiums. Having them in different namespace (and possibly not rendered at all) would cause problems (people surprised by their existance, edit wars to make them again paths). Leisure=track could be a secondary tag, but I think it is wholly unsuitable for a primary tag for these. It is not like they are behind a fence and not crossi general purpose trails.