There are a bunch of companies out there today that offer their products in a dual-license style, where you can download and use the GPL licensed version or buy the proprietary licensed version (often together with some kind of service deal) that you then can use without the "burden" of a GPL agreement. Popular known brands doing this include Trolltech/Qt (now Nokia), MySQL (now Sun), OO.o (Sun), Sleepycat (now Oracle) (Berkely DB is not strictly GPL but still copyleft) and VirtualBox (now Sun) etc.
It's perfectly legal for them to do this, as the company is the copyright holder of all the files, they can just easily re-release everything under whatever license they want at their own discretion. The condition is of course that they are in fact copyright holders of everything, that the parts they don't have copyright for are either licensed under an enough liberal license or that they can buy a similar relicense from third party lib authors.
It kills contributions from non-employees since doing a large chunk of code for these guys means that you would hand over copyright to a company whose entire business idea is to convert that to a proprietary license and make money from it. In a way you cannot do yourself since they can turn the GPL code into proprietary goods and you cannot. This may be a clue to why MySQL has less community contributors. The forced assigning of copyright over to a company could very well also be a contributing factor to OO.o's problems to attract developers.
Companies "hide" the truth about this and try talking customers into the proprietary license. I've worked a bit with Qt and the wording they have used have often given companies the impression that they have to pay for the proprietary licensed version to be allowed to use the product in a commercial product. I've had to explain to several customers that as long as they just adhere to GPL they can use the free version just fine without paying anything. Trolltech also has this dubious condition tied to their commercial license: "The Commercial license does not allow the incorporation of code developed with the Open Source Edition of Qt into a commercial product."[*] Needless to say, this will prevent companies from trying the open source licensed route first. I'm curious if they even have the legal right to make that claim.
This puts competitors at an arm's distance of course since no other companies can take the code and conduct business the same way. Of course this is part of the reason why they gladly adapt GPL for this. Lots of actions by these companies make me feel that they aren't real and true open source believers, but that they use this label a lot for marketing and for making sure competitors can't do the same as they do.
The GPL version is without support for customers in another push to drive them to pay for the proprietary license instead of the GPL one. Of course, it being open source lets companies going the GPL route to fix their own problems since they have the source and all, but the push towards the proprietary license also narrows how many customers that will actively contribute anything back since there's little chance they will do anything in a project with a proprietary license. I honestly can't see many other possible legitimate reasons why companies wouldn't do support for the GPL licensed versions.
I've not personally worked in any of these projects under such proprietary licenses, but I would love to hear experiences from people that have!
Obviously all this are not problems large enough to concern users. Quite possibly so because these companies do a good enough job and keep the GPL versioned versions of their software at a sufficiently good quality so that there just don't appear any forked projects that take the GPL version and run with it in a different direction. Another explanation could be that there are good enough alternative projects to go with if you're not happy with one of these dual-licensed ones.
A little related anecdote told to me by an MySQL employee (who's name shall remain untold). He described how they still haven't implemented a feature in MySQL that many people have requested, since they according to him don't want to cram in more stuff in the existing branch but instead are releasing it in the next major release (due to release in 4-6 months or similar). In the next sentence he explained how they already have it implemented in the closed version for at least one paying customer... Any (other) true open source project would've made that change available as a patch/branch in the GPL version for the public.
I'm pretty sure I personally would release my patches as open source only if I would change any code for any of these products. But yeah, that would mean that they would never get incorporated into their "real" products...