It is interesting to note that 16 of these countries rate in the top 25 for per capita GDP, the only exception being Italy (26th).
This may reflect the fact that people in these countries that enjoy a higher standard of living have more leisure time to do with as they please. It is probably safe to assume also that countries with higher standards of living have a greater number of clubs and organisations based around leisure activities such as sports clubs and hobby groups.
In respect of the top 2, Australia and USA, rural firefighter numbers most likely impact significantly on this stat.
Both of these countries have a very serious rural fire problem. USA alone has well in excess of 1 million firefighters of which around 75% are volunteers according to
Australia has large numbers also, for example in the state of Victoria alone there in excess of 50000 rural firefighters,see www.cfa.gov.au.