In the US we're supposed to have a very high threshold for what kinds of speech can be punished. For defamation, it is required that the plaintiff be able to show that there were damages.
The jury's awarding of $1 shows that they were not persuaded that there was any financial damage done to Mann by this think tank's actions. Their subsequent award of $1m in punitive damages shows that they were persuaded that Steyn's speech deserved to be punished.
That second part is what I'm not comfortable with. If they awarded him $1m in compensatory damages that would be one thing, but choosing the absolute minimum and then levying a huge punitive fine is a scary result for anyone who values the speech protections we have in the US.