I just finished this, so I'm late to the game, but.
I don't agree that the Humanity Firsters are supposed to be Tea Partiers. Unlike the Tea Partiers, the HFers aren't against government nor are they against paying taxes--they simply want their taxes to be used to only benefit Datum Earth Americans and not to subsidize the Long Earth colonizers. The HFers may be anti-Washington, but they're not libertarians like the Tea Partiers. If they were, they would be fully supporting the idea of stepping and doing everything they could to find a way to step to new worlds themselves, far away from gummint control).
What the HFers are are simply a broad parody of violent right-wing extremists in general. People who, being left out of the opportunity to share in the economic opportunities enjoyed by the more fortunate (i.e., the steppers), become violently jealous, paranoid, and use any kind of rationalization to justify their anger.