There is a very important consideration here that this opinion doesn't really touch on, but I think is invited down the road for future cases and legislation: Can you compel the speech of a third party to aid in exploratory evidence gathering (aka fishing expeditions) without a clear, well defined, and particular, cause of action at court to issue a subpoena?

In most classic U.S. jurisdiction, no, you cannot. Compelled activity or speech is generally frowned upon. The most important part of this case, IMO, was the Supreme Court constraining the Fourth Circuit's interpretation of contributory liability and attempting to turn the DMCA system into one for enabling those fishing expeditions.