I asked for a mug at my local suburban Starbucks and got a blank look in return for a few seconds while the request registered. Finally they found one on a shelf out of reach of all but the tallest staff member. There were 3-4 on that shelf and I've never seen anyone else ask for or use one despite the place being filled with seated customers at most hours. I also was unable despite 15 minutes of searching to select a mug on their online ordering app on iOS.

Think of the ecological impact if Starbucks just asked patrons whether they would like a to-go cup or mug when they order inside.