@kolev @AlexanderKingsbury That's the difference - if it's voluntary, it's a charity, if it's mandatory it's a tax, and we interpret those very differently (and we should).
If I don't tithe, then nobody will actually force me, I just know I'm not doing the mitzvah, and certain consequences may or may not flow on from that depending on G-d's will. If I don't pay my tax, I will be subject to violence by the State.
These are not the same situation.