They won't necessarily accept returns directly from someone who isn't their customer, as its possible for people to cause problems. For example if a user sends an S5 to Bitmain, but they purchased from a reseller. Bitmain says they will refund the item, the reseller says "nope" and ignores everyone.
Tldr if you're purchasing from a regional reseller, contact them if someone goes wrong with that sale. It becomes far too complicated when trying to make it a 3 way transaction.
No one is trying to buy from a re-seller and return to Bitmain for a refund!
I didn't say they were, but look at the problems some of these resellers put up as soon as you try and return anything. Bitmain can't force external companies to do anything, so can't get involved with what is essentially a 3rd party dispute. If you purchase from a reseller, your contract and sale is with the reseller.