Are there any other business that do this?

Microstocking is the only area of my working life where I've been a supplier of goods.  I'm finding it hard to imagine that a supplier of, say, DIY items to a DIY shop would accept receiving lower remuneration whenever the DIY shop decides to discount their items in the shop window.

This does happen 'quite' often, different industries more so than others, but it's a valid approach to 'moving more product', the supplier and retailer agree on a discount value, they usually split the difference (or even the loss) between them, then benefit later on when new buyers come accustomed to that particular brand.
In this case though, you could argue that iStock didn't ask you if you wanted to share the burden of their marketing efforts, however I'm sure they'd say that by agreeing to their contributor terms, and choosing to 'move your product' there, you've already agreed with it in principle.

I don't agree with the practice, but if what's good for the goose is good for the gander, then I can see why they do it.

Talking about 'deals', yesterday i received $0,84 for a medium download, i checked and all my other mediums are for $1,50-1,80 with here and there an exception being around  $1,20-1,30 (im obv. non-exclusive).
Seriously, $0,84 for medium? My smalls pay around $0,60-0,90...
I just was wondering what you think... would it also be the result of such a 'deal', or is it a mistake and i'd be best to contact Contributor Relations?
Have to watch your stats like a hawk these days 

thanks miss jsnover that matches with my calculations (which i dont dare to rely on because i'm seriously maths impaired). Must be a unlucky coincidence to have this new low (its the same $ as for a small dl!) happening at the same time as one screwup keeps following up the other.