Is respecting GPL too complicated?

I've had a bunch of hunches recently about companies abusing the licensing terms of Dokeos by officially declaring (but only verbally so far) that their product (being Dokeos, but not officially) is not open-source software. The GPL licensing makes it clear that the software derived from GPL software, or any alteration to a GPL software *must* be open-sourced, at least for the people it is distributed to. While I'm slowly starting to plan a series of legal actions, this is something I sometimes add to the bottom of my e-mails:

I'd like to take the opportunity to remind you that you are free to use the software, but that any alteration that you would redistribute needs to be made available to the people you distribute it to, and any application you would embed Dokeos into needs to be open-sourced as well. This is about the only restriction that applies to our software but we are very strongly watching that everybody respects that. Of course, we welcome external contributions and will almost always integrate it into the main distribution, avoiding additional update costs in the long run.

It's very sad to me to see that, with the amount of efforts we put into this software, some people still find ways to not respect the rules and selfishly profit from something that has been passed on to them along with responsibilities... This is clearly stealing intellectual property. While I understand that some companies might have problems in their business plans with respecting the GPL (because they develop their own software and don't want the source to be available to third parties), the rules of GPL are simple: either use it and comply to open-sourcing your code, or do not use it in the first place!