If it's a selling point for customers I don't see it as any issue. The dealer I bought mine from give "points" on anything purchased from the parts dept redeamable for cash, along with an initial discount on extra stuff bought at time of sale and another discount coupon you can use within the first year.
On my vehicles, the dealer I usually buy from also happens to offer free oil changes for the life of the vehicle on vehicles purchased from them. So for the last 200K miles on my S-10 oil changes have been free, the last 120K on my silverado diesel oil changes have been free, same on my chevy van.
IMHO it's all about good customer relations.
