It’s essentially what JerseyFrank has said. Traditionally, repair shops have made some of their profit on labor and some of it on parts, and most shops still structure their pricing along those lines. While that pricing model remains, the auto parts market has changed to where the general public can buy parts at pretty close to a shop’s wholesale price.
It’s only natural that people don’t want to pay full retail for parts if they don’t have to, so it seems to make sense to buy the parts at an attractive price and just pay the shop’s labor to replace them. This leaves the shop with three choices: refuse to do that, do that but adjust the labor from a “we supply the parts” level to a “you supply the parts” level, or do that for their “we supply the parts” price and miss out on their normal profit. The first choice doesn’t satisfy anyone, the second may be resented by the customer, and the third will be resented by the shop.
The root of it is that auto repair is typically priced on the assumption that the shop will supply all that is needed, and the breakdown between parts and labor is done for accounting purposes rather than with the intent that either component will be sold separately at its proportion of the combination price. Asking for labor only at that combination-based rate breaks that assumption.
Some shops will do the labor-only job at the lower rate, but grudgingly. It’s not a good business model. Refusing the job avoids the problems (see below), but doesn’t help the business any either. Taking the job at a higher labor rate can avoid the loss of profit, but does present other difficulties.
If the shop doesn’t supply the parts, it can sometimes be a bad situation for the customer and the shop. It may be the wrong part, giving rise to delays (sometimes with a car disabled tying up a stall) and the problem of who’s going to supply the right part, and how soon. If the part happens to be faulty, it’s not the shop’s responsibility, so the customer would have to pay the labor a second time to replace it. In a worst case for the customer, he’s caught in the middle of a shop that insists it’s a faulty part that they installed properly and a parts supplier that insists it was a good part that was damaged by improper installation – no warranty from anyone. In a worst case for the shop, the dispute goes to court where a judge deems the shop experts who should have been able to tell the part was faulty (though they never say how to achieve this miracle) and the shop is stuck with providing warranty for something they didn’t even sell in the first place.
An imperfect but often used, and often helpful, analogy is bringing your own food to a restaurant so as to get a discount on the meal. If that steak is tough, was it a bad piece of meat or did the cook botch it up? If you get sick, was it previously contaminated food or an unclean kitchen? Who’s responsible for the customer’s dissatisfaction?
Most of the better shops will generally discourage customer’s supplying parts, but will consider going along with it on a case by case basis. Factors involved include whether it’s a regular customer, the potential risk to the shop, specialty parts that the shop’s regular sources might not provide, etc.
So while it’s not necessarily a “dick move” in the sense that the customer is intending to hurt the shop in any way, it can be “dicky” for the shop nonetheless. My recommendation is to let the shop provide the whole job, and that way the shop can guarantee the whole job.