It appears that the mandate tax being stripped away actually makes the tax bill cheaper. Not really sure how, but the Senate is evidently just playing along with the CBO's screwy math. According to the CBO, killing the "individual mandate" tax will save money that the Senate can then use to pay for other tax cuts.
To me, killing one tax to help pay for other tax cuts sounds like a win-win. To liberals it's Armageddon.