"Interoperability is a very import facet of the government. Unlike any given business, wherein it could be argued that interacting with them is optional (you don't *have* to do business with anyone), interacting with the government is often not optional. So requiring the government to use software which can inter-operate with any software is rational, even required.
You seem to be missing one central point.
There is a huge difference between Open Source and Open Standards.
In the part of your post above, you state the argument for Open Standards. Interoperbility is good. Open Standards means anyone can access the data from any number of clients.
What the legislation you advocate does is mandate that the *client* be open source. Hmm. Why? Given a number of clients, does the determining step have to be that the client be Open Source with secondary consideration being given to other factors, such as functionality, ease of use, features, etc, etc?
You have legislated the *wrong* thing. Consider the example of Gnome and KDE. Both are Open Source. Both can access most of the same apps, but they are *still* working towards interoperability. These are two Open Source non-compatible apps for most of their lifespan. Will legislating Open Source help the person who locks into one with running apps designed for the other? You are still in the same bind in Open Source projects you are in now. You have to wait for the developers to add compatibility. The beauty of Open Source is that you could, if you wanted, add a third app that combines the features of the first two. I think the technical term is "kludge" for that third app.
If two apps can meet the same core functionality, which is the central point that everyone seems to agree upon, then it makes *no* real difference whether one is Open Source and the other proprietary. Each can have it's own functionality, look, feel, features, and ways of tying into additional apps as part of a suite, should they chose. You, the user, can decide upon which client you prefer based upon your own selection criteria. Another user can decide based upon their criteria.
All the Open Source clients in the world will not help if you do not have an Open Standard. And, no amount of legislating the use of Open Source clients will give that Standard.
Similarly, legislating Open Standards is about giving people choice in clients and there is no need to place an additional, unnecessary, requirement that everyone who wants to write, or use, the clients be given only a choice of Open Source clients.