I mean, I know we all oppose doing it, but what's the principle that justifies savings not being taxed that is consistent with our current system of taxation?
The Federal government taxes money you earn, then the state (usually) taxes it again. Any of it you spend get taxed a third time; if you save it, any interest it earns gets taxed. If you invest it, and make a profit, the profit gets taxed. If you buy real estate, the real estate -- which is just something you exchanged for the money -- gets taxed every year (and if you can't or don't pay the taxes, they'll sell it out from under you at auction, making sure they get 100% of what they tax before you get whatever, if anything, was left from the fire-sale price they accepted).
So we can't stand on the principle that the government shouldn't seize the fruit of our labor. We can't stand on the principle that they should only do it once, because we already permit double taxation even on income, and because we permit taxes on subsequent activity even after that. We can't stand on the principle that, at least once you own something and have paid all the taxes up to that point it should be yours free and clear: we continue to tax land you buy (and automobiles, at least if you want to take them off the land you bought). So accumulated wealth is already subject to taxation, in certain forms.
Progressives have been talking for years about a wealth tax, of which this is just a partial version. It strikes me that this form isn't that different from the property taxes we pay every year. Why shouldn't you have to pay for the privilege of holding a certain amount of wealth? There are lots of arguments, but are any of them consistent with what we already do?
If not, does that mean that the tax system we have is unprincipled? If so, does that make it unjust? Or is it fine to have a completely contingent system? If that, then, why oppose a wealth tax? It's just one more contingency.