I'd say it's quite simple: it's a retailer thing. On both PC and Mac, through their stores FD controls the sales without interference. So it's up to them to give both versions, the choice is their, fully. Same for Steam games, in fact. Unless specifically asked not to do so, Valve will make games that are pc/mac available for both platforms at no additional charge. For the record, while FD generously chose to give steam keys to their original customers, this in fact cost them money. A key redeemed means everything will go through Steam store and Valve will take its fee.
Xbox store is a retailer, they have to make money from the games they sale on their platform.