Yes, it's easy to see it as a cynical move, but it's worth noting that the 7 year rule is an ECB rule. Changing it to 3 years brings ECB regulations in to line with the ICC regulations on this matter. He's been pretty vocal from the word go about wanting to play for England, although whether that motivation is purely financial I guess only he knows. I seem to recall an interview where he said his reasoning was two-fold... he wanted the chance to play a the highest level he could, so he came to England, and the Windies snubbed him a few years back so he decided he'd try and play for England instead.
Seen a lot of him over here during the Big Bash, very likeable guy. And I've never seen a better last over bowler in T20! The thought of us having a top class genuinely fast bowler available is pretty mouthwatering