Running this site costs money, particularly over time as the storage requirements are quite large. Whilst those costs were initially met by generous sponsorship from the Ordnance Survey, more recently the site has been dependent upon donations. Granting commercial use allows anyone who runs the archive in the distant future to explore other options for generating funds, such as sales of montage posters.
Granting everyone those same rights actually protects the site community from exploitation), but do bear in mind that we only retain a screen-quality version of your image, and that under the terms of the Creative Commons Licence, you must be credited for any use of your image.
http://www.geograph.org.uk/help/freedom