They answer that in the NPR podcast the parent linked to. But it essentially boils down to the fact that taxes are constitutionally voluntary yet tax evasion is illegal.
I'm no constitutional scholar but I don't think the IRS is constitutionally prohibited from sending me a filled out tax form and asking me if it's correct.
"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."
