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!
The discussion is about warranty service, for which Bitmain can and would always repair or replace defective merchandise. I can't imagine a scenario where the issue a refund in lieu of repair or replacement during the 3 month warranty period.
The only remaining variable is when was the item purchased to determine warranty period, but that concern should be irrelevant during the first 3 months after the release of a new product since ALL items are under warranty at that point.