I think the problem with Button is that since the two uncompetitive races in Silverstone and Nürburgring, Button has been very cautious not to lose any more points. He realised the car is not as strong as the Red Bulls, so he would rather take the 2 points rather than taking risks for an extra 2 points. And i reckon you can't really blame him. This year is his 9th season in Formula One, and this year is the first time ever he has got a competitive car that is capable of winning the championship. Let's be honest, Brawn might not be as competitive next year, as a result 2009 is his greatest and possibly the only chance of winning a WDC. He wants the championship sooooo badly, his over-cautious is understandable.
So if you were in his position, would you really take risks after the massive point leads you have after the first 7 races? I wouldn't. If 2 points per race is all it takes, why risks it for glory?