The way I understand it, the logic is the same as with any scarce resource: to reduce demand you increase the cost. The target here, presumably, are really expensive health benefits offered free of charge to the wealthy. Logically, people who receive such benefits have no incentive to do a cost-benefit analysis, and may be eligible for paid elective surgery, etc. Taxing them or their employers for insurance over a certain limit may be a way to reduce the proliferation of such policies.

The trick is taxing employers for benefits. I have a small firm which provides the full freight for health insurance for our employees, because we think it's the right thing to do. Right now, we receive a benefit in the form of a tax deduction. If that deduction is limited, so we have to pay even more to provide the insurance, we may have to take a second look at whether it's a smart thing to do. Hopefully, a tax would be at a limit higher than what we provide, but it's a factor Congress has to consider.