NZ probably wasn't a great example, and 10 times was an exaggeration. But in general there's cricketers like Dan Christian and Johann Botha earning $500,000+ USD for a couple of weeks cricket. Then there's the SLPL and Big Bash, which pay on a slightly smaller (but still ludicrously large) scale.
A central contract with England is worth around only £350,000 for a years cricket. Pietersen is 32 with only 4/5 years left in him - who can blame a man for wanting to earn the money which will set his family up for life?