I never thought about that before but you're absolutely right. This is how the get around the Posse Comitatus Act.
However, the 2nd Amendment guarantees our right of self defense and even to form militias. They'll still put is in jail for using the 2nd Amendment because the Constitution and our rights are irrelevant to them. It was specifically written to be used against our government IF our government ever became oppressive. Arming IRS agents (an any other agency) is the very definition of oppression.
Keep in mind that the federal income tax has been in violation of the Constitution from day 1. The Supreme Court even ruled that it violated the 16th Amendment. So the left has been operating outside of the law ever since their guy Woodrow Wilson forced this means of controlling the people on us.
Fear is the only reason why most of us keep paying taxes, and when the people fear the government there is no freedom.