While some good comments have been made post-Qbix's statement that he has no problems with Value/Steam's integration of their code into DosBox, I think some clarification is in order.
Again, I'm going to reiterate my position that I think it is great that Value has chosen DosBox as it's method of distribution of these legacy games. It is the most frequently updated, stable execution of the task required of it. While Duke3D, Doom, etc. all have their esoteric releases, they are not nearly as uniform in configuration, stability or community support.
That being said, the purpose of Free Software, in both the beer and speech sense is communicated very clearly in the statements made by the Free Software Foundation in this document: http://www.gnu.org/philosophy/free-sw.html
In order for these freedoms to be real, they must be irrevocable as long as you do nothing wrong; if the developer of the software has the power to revoke the license, without your doing anything to give cause, the software is not free.
However, certain kinds of rules about the manner of distributing free software are acceptable, when they don't conflict with the central freedoms. For example, copyleft (very simply stated) is the rule that when redistributing the program, you cannot add restrictions to deny other people the central freedoms. This rule does not conflict with the central freedoms; rather it protects them.
You may have paid money to get copies of free software, or you may have obtained copies at no charge. But regardless of how you got your copies, you always have the freedom to copy and change the software, even to sell copies.
The above text is why Red Hat can charge for GNU/Linux distributions but in the same vein, CentOS can re-release system compatible RPMs that allow a previously Red Hat registered system to be updated indefinitely. Red Hat has held true to the letter of Free Software and CentOS is a, dare I say ironic, consequence of it.
So, from my reading of it, to link/patch/encapsulate a DRM wrapper around a GPL application is clear front to the very meaning of Free Software.
So what does this all mean really? It means that copyleft is amazingly fascinating and fluid. I'm personally in general agreement with Qbix. They've included the GPL license. They've included the DosBox source code. They've given proper attribution. To me at this point, a Steam integrated wrapper is trivial and Steam has indeed satisfied their part of the bargain. Does this stick to the pure letter of Free Software? No. Does it accomplish the goal of Free Software? Surprisingly, yes.