For a start, they should get enough ranking points this season not to require an invitation. But even if they didn't there's no reason why they shouldn't be invited.
1. Firstly, they're a brand new team. Have a look how 'superteams' like US Postal, T-Mobile or CSC did in their debut Tour (Clue: it was worse than Sky). A white jersey wearer, a second and two thirds on stages and two guys in the top 20 isn't that bad. And the Tour's not over yet.
2. Other teams have performed worse. What have Garmin, Cofidis, Milram and AG2R done? (I'm quite well aware two of these are French). Even Liquigas haven't really shone, for an Italian superteam.
3. As has been pointed out, they represent a big (largely) untapped market.
4. They are the personal project of James Murdoch. This is the heir apparent to the third largest media empire in the world. As cycling runs on publicity, this is a good person to have onside. Plus Sky are one of the world's best and most innovative sports broadcasters in the world. Sponsors like this need to be encouraged and are useful to ASO.
5. I don't see any better teams that have missed out. Personally I don't see Vacansoliel's suicide attacks for fifteen minutes as impressive.
5. There's a good chance of Caisse d'Epargne and Milram, at least, won't be around next year.