Why do retailers argue for the supplier and not the customer?

This is a phenomena I come across all the time. When querying a guarantee period or returning a product the retailer invariably moves to defend the manufacturer/supplier by aggressively stipulating warranty periods etc. Surely the customer should be the retailer’s main priority. It’s a complex question I realise based on buying power, geography and relationships and not all retailers are like this. It’s a pleasure when a retailer says “we guarantee our products and please return if any problems” with no reference to a manufacturers guarantee period etc. One gets that feel good factor and will undoubtedly return for future purchases. Richard Potton