Because that's a problem for Anderson Silva or GSP? They're not getting the big money, or the high-end endorsements or drawing in the PPV percentage bonuses, right?
Get real. A loss to Chael at 151 (however unlikely) would've done very little to his overall "brand" in the long run. May even make him a better fighter, like GSP's and Velasquez' losses. In the end, even if he lost, most people wouldn't take it seriously, since it was such short notice (though he did have a full camp for Hendo) and Jones would most likely get an immediate rematch (i.e. they could've fought at 151, Chael somehow wins, and they set up TUF to hype up the rematch). Jones wins that and he's back to where he belongs, with only a slight hiccup along the way.
In a way, yeah. Fighters fight an average of what, 2-3 fights per year? Fighters generally have a limited number of years in which to make their money before demand/ability start declining (the so-called "Prime" years). The fewer fights one takes in that time, the less money there is to be made. Of course, you can argue that by not taking the fight now, he's delaying the physical toll a fight would take on his body and thus, extending his career by one fight (but we know that's not the only issue with fighters as they age).
In the end, Jones had every right to decline the fight at that time, but that doesn't make him blameless, nor does everyone have to agree with his choice (or think it was the proper thing to do).