Some independent shops make a bit of profit from the parts they obtain from the dealer, or they don't like the liability for doing work with unknown parts - if it doesn't work right or fails, they are in the liability loop.
There was a long discussion on this a couple of years ago - if I recall correctly some shops discount their labor a bit and balance it out with the parts profit. Other shops installed the customers parts and were expected to make them work if there was a problem with the fit or finish, and yet do it for the same price as if the part were factory OE and fit without a problem.
I remember that the consensus from the techs was to ask your tech about the policy before bringing him parts, otherwise it is a little like taking your own food to a restaurant for them to cook, and reserving the right to complain about the quality or how it was cooked!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E