What would the government try to hide by charging in and killing them?
Sixteenth Amendment:
The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.
They know this, they are fully versed in the law(tax protesters always are). They refuse to admit it, they refuse to submit to the law like everybody else, and eventually they will serve their time.