Their conditions seem aggressive to me and I would never want to fall into such claws. I rather earn less but remain free. And how can you guys be sure that the earnings they are currently promising for exclusives will stay that way?
Can you tell me where did you read this ? Or who told you this ?
As far as I know istock did not promise anything to nobody, except maybe to some special guys with whom they have a special deal anyway. 
It's not a secret. iS pays a higher rate to its exclusives. For now.
http://www.istockphoto.com/help/sell-stock/rate-schedulebut as Einstein suggests, they have broken promises to exclusives before, and changed the goalposts on who earns what percentage and they have taken away most of their 'inducement' perks, so there is no guarantee the rates are carved into rock.
Back in the day, exclusivity was a valid choice. Today, congratulations to any newbie who can manage to get the 250 dls necessary to even have the choice (though the 50% acceptance rate must be achievable by everyone nowadays, since the standards dropped unbelievably).
However, you're only 'locked in' to exclusivity for 30 days, unlike some other agencies which lock all or some images in for three or six months.