Because I expect to sweat and if my back is a little sweatier because of a pack or messenger bag it makes no difference to me. I'm going to shower at work regardless.

Triple digit temps only happen once in awhile here and when they do I take it easier on the way home than I otherwise might. I'm not going to be out there for hours and hours on really hot days. If I were I'd probably take more precautions. Frankly it's never been a big deal.

Even when it's -20 my head will be sweaty by the time I get to the office. I'm working hard enough that my hands sweat on most winter days. Only my feet will be cool usually. But it's all fine because I intend for my commute to be a workout. I'd be more bothered if I didn't sweat at all.