like Bill mentioned the customer while has more rights then the repair shops do if things get to court I believe it has to be stated in your warranty.
If you mandate oil change intervals or cooling system maintenance and other standard service items must be done by a certified shop that has a biz licence tax number etc that takes care of back yard guys doing stuff himself..
As far as warranty repair issues like if something happens to an engine I believe it must be stated in the warranty that permission must be given by the shop 1st before any repairs can be done where they will hold the shop responsible for the cost That said it must also be stated how much time is allowed to be billed for by the hour according to Mitchel flat rate book or similar I do max $35/hr to cover the cost of basic labor cost not the full on shop rate.

You have to protect yourself in the warranty fine print as best as you can or you can find yourself in it hip deep with a goofy customer that figures your ruined his life for a day or two.
Things like oil changes must be done on time to your requirements and no owner service is permitted and customers must provide a paid and dated invoice by a licenced shop with mileage recorded on the invoice is the big thing as most people do not do it when they are supposed to.

A trick I do is with a permanent marker I date the  1st filter when it leaves the shop and check that if there is an issue.
I had a guy a few weeks ago have a bear go out and complained about noise. The 1st thing I checked was the filter and guess what in almost 2 years he never changed the oil or filter but when he showed up at the shop there was clean oil in it but a bad rod bearing from a timing adjustment that was way off ( 48 degrees at 2500 )where it left the shop the 1st time.
I asked the guy to produce the oil change records required by the warranty and all he could say was it was changed till I showed him the filter that was dated and recorded by a picture that is dated when installed.

Just saying the warranty has to cover you ass from cheats and dumb ass's that try to get something for nothing when its their fault