[logfromblammo]

Given that the IRS can operate under any rules that the government pleases itself to create, regardless of any connection to reality, I don't think anyone there even attempts to discern whether a given rule is actually generating revenue or just fruitlessly destroying commerce.

As you say, anyone with enough cash available would always prefer settling with two trades using money instead of one barter trade. So really, the barter tax is only levied on people who don't have the money to pay it and on tax avoiders. The tax avoiders find another loophole; the poor people remain screwed.