Zillow has never sold a home, the ONLY reason Zillow has a listing on its site is because an AGENT has already found the listing first.
IN regards to commission, the BUYERS pay the commission. Just like in every retail sale.
AND YES, hiding if from buyers is a big problem for sellers. The commission is not hidden from sellers, its hidden from buyers.
Some sellers will offer very little for the selling agent (buyers agent) while some sellers will offer alot. These amount effect the selling price of home and the net of sellers.
Source: Inman news