All of this is intentional. An action like the one posted about should create major in-realm trouble, and trouble is always good.
What about limiting the rate of change? Taxes the Banker imposes on duchies can only go up by increments of five percent a day (though that's still bugged if I remember aright) but taxes on the realm can move arbitrarily? I'm all for Bankers stealing tons of gold, but there should have to be some skill involved rather than thirty seconds before turnchange on tax day setting it up to steal tens of thousands of gold.
