Why do people get all up and arms about product placements in Tv shows and movies ?

Also sometime ago some people became infuriated because some theaters chose to start showing commercials prior to the show time. Would they prefer just a blank screen like the old days ?

Why would people prefer that characters in shows/films use generic looking products rather than name brand ?
Maybe in both instances,it keeps the prices down.
I would rather see the products incorporated into the story line rather than for the show to be interrupted every few minutes and edited for time to fit in more cheesy commercials.