You can opt out of partner deals at BigStock if you wish. There was a thread a while back on the BigStock forums about this deal.
They're licensing images for high prices, not selling prints (at least that's what was happening back when this started). The problem I had was that the price difference was so high, for the same images that sold for less via the BigStock site that I felt any buyer who figured out what was going on would feel ripped off.
I found out that this was happening after I got a $7 commission on a sale - I think the image sold for $69 or $99. I still think the whole thing is a really bad idea, but then I opted out, which was good of BigStock to implement for those of us who didn't like the deal.