When I read things like:
I downloaded Geogebra a million years ago and recognized immediately its value as a free alternative to Geometer's Sketchpad...
You are free to copy, distribute and transmit GeoGebra for non-commercial purposes. Please see the GeoGebra license for details.
This is not an accurate description of GeoGebra licensing. First off, there is no GeoGebra license. According to the more detailed licensing explanation, "GeoGebra's source code is subject to the GNU General Public License," and "All GeoGebra installers, language files and documentation files are subject to the following Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License." In this combinations there are times that commercial distribution is allowed, and circumstances when non-commercial distribution is disallowed.
This is essentially a non-commercial variant on the old proprietary installer strategy that Linux distributions like SuSe used to use. The idea was that the distribution vendor would keep the OS installer proprietary, thus preventing people from legally redistributing the install disk, even though it was 99% free software. The thing is, this simply turned out to not be a competitive advantage, and I'm not aware of any major Linux distro that still takes that approach (including SuSe, although SuSe and Red Hat have proprietary versions via different approaches).
For an individual application this approach appears to violate the GPL itself, however:
For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable.
Beyond that, I assume that the reason people apply these kinds of licenses is not to keep small-time teacher/consultants from passing out CD's at conferences (although technically, it probably should) but rather to prevent a big corporation from co-opting tha application. In this case, a vendor who really thought they could make some money off selling or distributing GeoGebra could simply write their own installer, documentation and relevant translations, which may not be much of an cost, depending on the circumstance. For example, if I wanted to distribute GeoGebra with a textbook, these would be small costs relative to the whole project.
What this kind of licensing actually accomplishes is creating enough uncertainty that most people won't want to modify or redistribute the software on a large scale. For example, the Debian project is one of the main gatekeepers for free software and the basis (directly or via Ubuntu) of most of the government-sponsored educational Linux distributions used around the world. Here's their take on the situation:
Some weeks ago I did speak with geogebra authors to clarify geogebra installer license. At the end, IMHO there are somme (sic) License issues not debian compliance.
People, corporations and governments trust Debian because they don't put up with ambiguously licensed software. You can build on it without worrying that some sloppy licensing bomb (like GeoGebra violating the GPL by excluding the instaler) is waiting to go off in there. And that's why things like GeoGebra don't get in Debian and get into the hands of fewer kids than they would otherwise, with no apparent offsetting benefit to the original developers.
I think Dr. Geo is the true open source alternative to Geometer's Sketchpad, but it is quite possible there is something better I'm not thinking of.