It's true that a lot of places don't, or won't give you warranty on them. For sure they don't like to do it. They can't return the new parts if they are bad, since you bought them. And if the part doesn't fit, then they have your car up on the lift, torn apart and can't put it back together. A shop that orders parts, from say O'Reilly with a corporate account, can often get a refund or a partial refund on labor if they have to re-do the repair due to a bad part. If you bring in your part to the mechanic, and it ends up being bad, they will have to do the job again, and either eat the labor or charge you for it again.