They can...they hadn't used to. But people sold items for peanuts with stupidly high postage so they charge fees on postage now.EBay take a slice of it. EBay can't take a slice of the postage cost. So it's beneficial for them to bump up free postage in the listing.
So if you knock two quid on postage and do the item two quid cheaper. You'll be a few pence better off.