Note that this is only talking constitutionally. The government is free to pass laws that disallow such gathering, or simply not pass laws that allow it. IIRC

Note that this is only talking constitutionally. The government is free to pass laws that disallow such gathering, or simply not pass laws that allow it. IIRC collecting mail is not in fact legal without a warrant, and only photographing the outside (which anyone who handles it can see) is allowed. I do think the fact that any member of the public can collect data legally means the government should be able to as well.