If they didn't nobody would know about it. It's not like Amazon has a big list of email addresses of people who have Secret data that they might want to put on the cloud. Even if they did have a list of government people like that, most of the people interested in this service are contractors who don't officially work for the government.

The government doesn't work like those spy movies where everybody knows everything the instant it happens. It's more like a big bloated corporation with thousands of subcontractors and generally lousy communication all around.