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They used to, but now do it themselves and then keep the database up to date with Android phones. This is why Skyhook is upset and suing:

http://gigaom.com/2010/09/15/skyhook-sues-google-in-a-locati...

well, the same is true for Apple who are now collecting location/wireless footprint themselves with iOS devices without Skyhook.

Yes, it's significant to note that Google (à la Street View scanning) is not the only party that's made a concerted effort to collect them. Whether Google's purchased (or absorbed) such data from others, I don't know.

I speculate that further routes for collection may be available. E.g. a user uses a wi-fi connection with a device that has geo-location turned on, and "Bob's your uncle".

I didn't know you could (legally) earn money for wardriving.

It's just 21st century map-making. Instead of plotting the location of physical landmarks, you'd be plotting electromagnetic landmarks.

I'm sure pioneer map-makers got in trouble for plotting perfectly visible stuff that was located on private property.