Speaking generally, one issue a site has to address if it takes direct submissions is inspections - using someone else's API is effectively allowing them to skip that often messy part of the business.
Unless a site seems to be generating sales in any reasonable quantity or with regularity, why include them?
And as far as keeping the images to save you uploading, I highly doubt they'd be able to do that given the legal agreement they have with their source (DP in the case of Shotshop) and/or the technical details of the partnership - sometimes the distributor only has the thumbs and the main agency has the high res unwatermarked version.