Warranty work is filler work for when they don't have paid work.
Warranty work is typically little or no margin and they have to get approvals thru thru the manufacturer. On the other hand, if someone shows up gives them a credit card and says fix it, they can price it to make a profit.
Plus your warranty work likely isn't going to another dealer (or mobile tech). On the other hand, the guy paying cash, can go elsewhere if they can't fix it in a reasonable time period.
Basically, warranty work for minor issues is overrated.
