Being "open source", or complying with the GPL license, it cannot be sold. It can be given away with the source code such that it the person you gave it to were to use that source code and create changes and/or additions, they would be obliged to give those changes back to the community as open source code in itself.
These people are selling the "media only" - i.e. the disk it comes on. They don't have the right to sell the software - it's a very loose interpretation of the rules. Linux is another prime example - sold for US$5.00 because that's the cost of the DVD media. At least you GET the source code with most of these distributions - unlike Stellarium.
Anyhow - Stellarium is, in my opinion, very slow, unstable, clunky and does not cover the southern delights so well. Other than that it's barely OK...... but other than that I guess it's free!
