Well for one, because money tends to follow popularity... not the other way around.
Look at the Herman Cain campaign, he was cash broke until he hit a chord with the people, then the money kept pouring in. Then after it started pouring in, no amount of money could save him from sexual assault accusations.
The effect of money on campaigns is GREATLY overstated and there is money on EVERY side of just about EVERY issue. So nobody really needs to be "bought off".
I would suggest that cases of politicians changing positions after being elected has little to do with campaign money and more to do with lieing to get elected. As can be seen by all the little things that essentially have no "buy off" value.
Take Obama for example. Refusal to recognize the Armenian genocide and his about face on FIZA and just about every Bush added patriot act abuse.
What stops it, is people simply electing someone else when they do too much crap. It's pretty simple.
It's generally a needed system though to prevent short sighted, short lived reactionary beliefs from overwhelming the election process and causing massive harm by being implemented... although we aren't totally immune from that as we've seen some legislation passed due to the whims of the moment that largely ended up proving damning.
Like the Patriot Act and Stimulus programs. When the Democrats and Republicans respectivly were eseentially forced by public sentiment to go with the others dumbass idea. (Well, to be fair the stimulus program was half bush's idea, it's just the rest of his party knew better.)
I'd suggest Reading "Freakanomics". In general cause it's a good book, but in particular for the actual effects of money on campaigns.
