A copy of the decision is here. Two other federal district courts have previously upheld the individual mandate against constitutional challenge, and at least one suit remains pending.
It’s often claimed that the individual mandate is a necessary compliment to the provisions of ObamaCare banning denial of coverage based on pre-existing conditions and so forth. The idea is that if an insurance company can’t deny you coverage once you are already sick there is a strong incentive not to get coverage until you are already sick, which leads to a death spiral (that’s a technical term) of increased insurance prices and lower levels of coverage.
However, as Paul Starr noted back when the bill was being debated, there are ways of dealing with this problem that don’t involve a mandate:
The law could give people a right to opt out of the mandate if they signed a form agreeing that they could not opt in for the following five years. In other words, instead of paying a fine, they would forgo a potential benefit. For five years they would become ineligible for federal subsidies for health insurance and, if they did buy coverage, no insurer would have to cover a pre-existing condition of theirs.
The idea for this opt-out comes from an analogous provision in Germany, which has a small sector of private insurance in addition to a much larger state insurance system. Only some Germans are eligible to opt for private insurance, but if they make that choice, the law prevents them from getting back at will into the public system. That deters opportunistic switches in and out of the public funds, and it helps to prevent the private insurers from cherry-picking healthy people and driving up insurance costs in the public sector.
For whatever reason, the Democrats choose not to head this advice, and didn’t include any alternative to the mandate in the bill, even as a fall back measure. This means that, if the mandate is ultimately found unconstitutional, there will be nothing in the law to prevent the “death spiral” scenario. Granted, this can always be passed in the future, but this may not be as easy to do depending on the political environment at that time. Why the Democrats didn’t do this is a mystery to me.