Again, it's completely reasonable given the world we currently live in - something which cannot be ignored, especially when negotiating expensive contracts.
And even if you do ignore the outside world, over the last few years Hamilton hasn't really proved that he deserves anything more than parity with Button when it comes to his salary. It's all very easy to point to the current championship standings and claim, as you have, that Hamilton is 'destroying' Button, but by that same yard stick, Button destroyed Hamilton last year.
On the right day, he might be very bloody fast, but he's woefully inconsistent and incredibly volatile off-track. He's the complete antithesis of Alonso and Schumacher (in his prime), and it's that quality (coupled with speed) that really warrants the 'silly' money.