we believe: it is unethical for government to enforce proprietary software licenses, which prohibit people from sharing with each other
That's a little bit extreme to me.
Overall, the letter is not that convincing. They highlight the fact that with free software, costs go down because you don't need to track how many licenses you are using. True. But on the other hand, you need to track the modifications you make and make sure you follow the redistribution requirements. So honestly, it's a wash: either way, you need to dedicate resources to enforcing the terms of the license.
The government doesn't do a lot of external redistribution of software that I know of, besides things like VistA[1], which is free software itself. For the vast majority of the software the government uses, the license enforcement costs would be zero. For the rest, the cost would be less than the corresponding proprietary application.
But why can't free software compete without the government mandating it's use? Without licensing costs, it should have an advantage. The answer is that free software is often unresponsive to user's needs, and telling users to scratch their own itch is an example of that. Open source has achieved huge success among developers, because as a community of developers, they deeply understand those needs. It's been much less successful at developing products for non-technical users because it doesn't understand them, and has no strong incentives to try.
That's a little bit extreme to me.
Overall, the letter is not that convincing. They highlight the fact that with free software, costs go down because you don't need to track how many licenses you are using. True. But on the other hand, you need to track the modifications you make and make sure you follow the redistribution requirements. So honestly, it's a wash: either way, you need to dedicate resources to enforcing the terms of the license.