That is not true. Some federal jobs (not contractor jobs, actual employment by the federal government) can be taken by non-citizens. Others cannot. Competitive civil service jobs cannot. Higher-up jobs with policy-making authority cannot. But several lower-level jobs are open to non-citizens.
The jobs that are available to non-citizens, however, are generally not open to citizens of all countries, only to citizens of countries in a current military alliance with the US (e.g. NATO countries and some other countries with good diplomatic ties to the US).
But don't take my word for it. Here's what the government has to say about it: 'http://www.usajobs.gov/EI9.asp