Because we can't collect pressure readings in the background (apple doesn't allow a background mode for pressure), so it would just be a waste of the user's battery to be constantly taking their location.
No way, my old N4 had a barometer? Wish something like that would be more exploited/publicized, I'm a data nerd and would have loved to play with apps which use it. Good to know, the N6 should have one too in this case.
Why can't I allow WeatherSignal to access location services always?
Settings->WeatherSignal->Privacy->Location Services looks like this: http://i.imgur.com/BeOcOLP.png
when it should look like this: http://i.imgur.com/kAOB53B.png
The option "Always" is missing from WeatherSignal settings.