I'm pretty sure that Magnusson-Moss did away with the requirement for dealership service for warranty coverage.
In the old days the dealership could require that owners have their oil changed at the dealership to maintain the warranty. It wasn't uncommon for a pre 1975 dealership oil change to cost $50 or more. Failure to do so could, and often did, result in the car's entire warranty being voided.
It now falls on the owner to prove that maintenance has been maintained in a manner consistent with manufacturer specifications.
In other words, the dealer can't, as was common in the old days, cancel your warranty totally because you have a set of non GM or Chrysler but totally OEM spec spark plugs installed.
Here, check out this link and scroll down to "tie in provisions."
http://www.ftc.gov/bcp/conline/pubs/buspubs/warranty.shtm#Magnuson-MossHere's the applicable section: "Generally, tie-in sales provisions are not allowed. Such a provision would require a purchaser of the warranted product to buy an item or service from a particular company to use with the warranted product in order to be eligible to receive a remedy under the warranty."
It's no coincidence that places such as Jiffy Lube and Kwik Lube Express, as well as a bunch of other businesses just like them, came into existence after 1975's Magnuson Moss act.