You can subpoena a third party, but the third party can oppose it. Assuming that the challenge to the subpoena is rejected, courts will generally require the l

You can subpoena a third party, but the third party can oppose it. Assuming that the challenge to the subpoena is rejected, courts will generally require the litigant to reimburse the third party for costs incurred in responding to the subpoena. While federal courts technically require the responding party to shoulder the costs, as a practical matter only governmental third parties are still required to respond to subpoenas out of pocket. In this case, Apple sought tons of confidential trade secret information (basically, sales, revenue, and traffic data for every product ever sold on the Steam store for the past 6 years, plus advertising and other revenue related to Steam, plus other side channel revenue), and is it likely that most of the subpoena will be quashed, considering that Valve did respond to the reasonable portion of Apple's subpoena.