Same as ebay
Ebay have the same rules, if you want to get good rates for a shop there they insist you 1) list your whole product range and b) keep price parity with your own site.
It's a simple commercial decision - you want to list on ebay / amazon because there are lots more eyeballs there, it's a busy high street and it will bring you customers you wouldn't have had.
But you want them to come to your own site the next time, because you don't want to lose money on listing fees.
The easiest way of pulling people to your own site for their next purchase - be cheaper, as simple as that.
What's in it for Ebay and Amazon then though? They get one paltry commission and lose a customer for that type of sale next time, not worth it.
With these sorts of rules in place it's win win for the ebay and amazon type sites. They get a bigger better product range (courtesy of your hard work), they get some free money from your sale and they don't have to worry about customers leaving their site.
They have to wonder though - what is in it for the 3rd party seller to use ebay / amazon under these conditions? They get a bigger market for sure but at 15% it's a steep price to pay with very little hope of ever pulling them to your own site.
Ever heard the one about the golden goose?