Article here.

Agreements with private firms for goods and services are not the only sort of contracts the federal government negotiates. The contracts it negotiates with its unions, for example, dwarf in financial scale the typical business contract. Those public-sector unions, as you may have heard, are rather lopsidedly Democratic in their political sympathies. No surprise, then, that they are to be exempt from these restrictions. Likewise, organizations receiving grants and other support from the federal government — public dependents that unsurprisingly tend to favor a large public sector and, consequently, Democrats — remain immune.