There are a few legal catches to this... they're only being asked to reveal the names of the people who have released the information to Apple Insider, who were employees in violation of trade secret agreements. This isn't about violating Free Speech, its about prosecuting people (employees) who violated those NDAs and trade secret agreements.
Free Speech is protected, the part that is not protected are the original sources, who are breaking current, legally-binding confidentiality agreements to reveal the information, and the fact that the web sites, by publishing said information, are also in violation of the Uniform Trade Secrets Act.
Apple had sufficient evidence that the information wasn't just accidently leaked to convince the court that trade secret laws had been violated and for the subpeonas to go forward.
There is a
great blog entry on this exact topic. Highly educational reading.