I'm just now joining the thread. I would say force is too strong a word as it brings to mind lawsuits and court orders. However, for commercial projects that I work on that use open source software (non-GPL), I would certainly not go out of my way to list all the software I was using without being compelled to by a license. The simple reason is that there is creativity in aggregating software just as there is in writing code. If a competitor had in their hands a complete list of technologies used to implement an enterprise product, it would certainly not be equivalent to having access to our source code, but it could certainly provide insight into how some of our sexiest features were composed. On the surface, the competition argument in this case doesn't seem that compelling because it's a governement site, but in reality the government contractors are competing in the marketplace like any other software consulting company.
Of course I could start another thread here about how people who use open source should open source their software. I think the reality of that is that most businesses are not open to this. Although none of the applications that I have worked on professionally are open source, I have contributed several patches and bug fixes to help improve the underlying libraries the software depends on. I still sleep fine at night.