The reason we don't allow linked photos in the Member Marketplace is that photos represent condition. Linked photos can be de-linked post-sale and when what was represented as "mint" condition, turns out to be a "scratched, bent, damaged" item, but without the photos, it's: "he said, she said." We want to our Marketplace to be as fair and square as possible to protect the buyer and the seller, so we require loaded photos for all Marketplace FS items.
Hope that answers the "why" question.
Pro dilectione Dei et patriam