Good question. I would say that those listings with "global location" in the title are open to remote working (like the BTRFS developer or Rails maintainer I list above), and their affiliation to a geographical office reflects more the budget that the team manager has been allotted (salary for a US position != salary for Czech Republic). Try making a search using "global location" as keyword -- I reckon the "telecommute" toggle in the careers page doesn't work.

Another thing I can say is that even if not listed in the job ad, remote working is offered to very solid candidates if asked. Example: an established kernel developer can easily ask for telecommute; a junior less so.

If there is a specific position that interests you, send me a mail (address in my profile) and I can ask the team leader of that group if the specified location is negotiable or not.