It may surprise you, but government offices do actually pay postage.

There are several reasons, including issues of separation of powers, but the most basic one is that the USPS is supposed to actually get paid for what they do. If government agencies could just dump stuff on them without paying postage, how could the Post Office possibly deal with budgets.

I think part of Nate's argument is that only a fraction of those receiving the covers want them. As such, there would need to be fewer made.

These are made-up numbers, but consider: Suppose their are 1,000,000 people in their target audience. Suppose it costs $1 to produce and mail a cover. To mail to everyone, that'd be $1M.

Now, suppose that they only sent them to people that wanted them. Let's say that's 100,000 people. Therefore, it would cost only $100,000 produce and mail them. Ah, but what about the cost of mailing 1,000,000 postcards plus the cost of having to warehouse these things longer plus the cost of having to have staff to process the postcards? Well, turns out that those costs don't add up $900K. They'd be more like $300K. Which means it's still cheaper to only send to those that want.

(I know this because I used to work for government contractor that was in the business of warehousing and mailing out items on behalf of the government. Trust me, we didn't send our entire inventory to every household in the country under the mistaken notion that it'd be cheaper to just do that).

The question is: does it recruit more people to have a million of these things out there, instead of 100K? That's what they're banking on -- people seeing these book covers and thinking "hey, the military is cool to send out free stuff" and then signing up.