Should LDS members earning less than $30k be exempt from tithing? Not a loaded or leading question, just curious how you would answer this and distinguish the differences.
No, they shouldn't be exempt from tithing. But it's not an equivalent matter. Tithing is (in my opinion/the LDS view) a commandment from God, with a flat rate mandated by revelation. And people are promised blessings for following the commandment. Taxes are not a commandment from God, except via the "render unto Caesar" thing, and there is certainly no such thing as a flat governmental tax rate revealed in the scriptures. And no promised blessings for paying a tax (except the blessing of staying out of jail ;-) ).
Actually, when the United Order was in effect, you could say that there was perhaps even a negative tax rate for poor people because they got back more than they gave to the government.
Anyway, another consideration is that poor LDS people also sometimes get helped via fast offerings. When in that situation their "effective tithing rate" (my phrase, not God's) would be lower than 10% if you consider the money they give up combined with the financial assistance they get back. So even in the LDS church, tithing isn't as flat as the flat tax proponents might want us to believe.