Perhaps CrimeStoppers' agenda is to stop crime.
And also to help apprehend culprits. Rewards by the police seem to be similar (if indeed rewards are distinct).
The lower amount offered by the police is an incentive, but not so high as to create a media frenzy of false sightings.
For a desperate family, a reward keeps a high profile, particularly when there may be a high chance of finding the missing child / adult quickly. A huge one increases paper sales or general media advertising, positive PR, etc.
However... how much of this supposed £2.6 million reward by various benefactors is actually cash-in-bank ready to be handed over as opposed to pledges?
http://en.wikipedia.org/wiki/Reactions_to_the_disappearance_of_Madeleine_McCann#RewardsWho would be legally responsible for this alleged £2.6 million announced in the press? What about the amount pledged by the now defunct News of the World (£250,000), for example? Have pledges been withdrawn for any reason? Negative press assuming the parents were guilty?
Other benefactors may simply not have the same means that they did in 2007 when they first pledged.
The issue might well be quite complicated.