So is the one by me. When I asked the barista why, she said it helps keeps the fruit-flies away from the bakery items. I just bring a sweatshirt with me when I go.
When I'm in the states, I bring a jacket in the summer to starbucks.
I actually had a problem with certain starbucks in Beijing that don't have adequate heating in the winter...my feet are just hurting after 30 or so minutes. I've mapped out all the stores with decent AC and heat now, though I switch stores seasonally.