There are reasons why I would mask in a vehicle: expecting passengers, returning from shared spaces and want to prevent hand-face contact.

I've puzzled about outdoor use, but I'm not getting off my bike to ask anybody. Best guess is they're subject to early (mis-)information about virus spread or they're at risk and taking all precautions. They may have allergies and are just avoiding pollen. What should I do about it? I did some extra research and confirmed wearing a mask outdoors probably isn't beneficial to myself or others so I don't. There is no harm from them choosing to wear one.

It's a practical tool to prevent spread of a respiratory virus. Trying to ascribe political motivations to every non-sensical example of mask wearing probably says more about your concerns than whatever motivated them to wear a mask.