While I see both your points, I think it's fine the way it is. In all reality, there will probably be (at this time) three International teams. Aus, Kiwis and GB. The Kiwi's and Aus, under your idea, would probably be able to feild a team but GB might struggle (if you're only picking players that play sevens and those who actually want to play rep matches - some don't). If people can make themselves available for other teams, then why not. It's only a game and show's the spirit in which people are willing to play it.
As for only being eligible for the country where you have lived all your life - not all people live in one country all their lives. And you'd be excluding some quality posters with it.
When it boils down to it, does it really affect how the world turns if someone born in Australia to English Grand parents who doesn't make the Aussie team then plays for GB?