[Freegis-list] Spatial Data Licence Question

[Brett Matson]

Hi everyone,

This may be an stupid question but it needs to be asked.
I understand why source code must be protected under the GPL 
or equivalent license, but why is it considered acceptable 
to release spatial data as public domain? Couldn't the same 
problems arise with people making data proprietary after first 
modifying it? Is there an equivalent license that can be 
applied to spatial data if necessary?

Thanks

Brett