Its called upselling to make money off of you since you are only in there for the free maintenance.
The Service Adviser gets a commission based off of any service or part they can sell you.
Then the Tech gets his piece of the pie by getting to do the job, If he is hourly he is paid regardless for the time he there, But if he is Flat Rate which means he gets paid for the set book time for that particular job, so if it takes him 20 minutes to do a job that pays an Hour he makes more money off of that job.
Depending on the dealer the Service adviser will pressure the Tech to find something wrong to sell you the work while you are there for the free service with stuff like the fuel injection cleaning, brakes ,ect,ect..
Or the Service adviser will just make crap up to sell you like what they tried with you.
They never look for warranty work because the Adviser doesn't get a commission from it, And the Tech has a lower amount of time set to do the job which will pay him less $ or make him work harder to earn his pay.