No, it's not volunteering, at least not anymore.

Subpoena is legal Latin for "under penalty," because noncompliance with a subpoena carries a penalty.

Originally, it was an information request from the feds, and Reddit refused. Then they escalated to getting a grand jury subpoena (which means they got a bunch of normal citizens to agree that the information was relevant to a criminal investigation), so now noncompliance carries a penalty.

Reddit notified the users, who hired their own lawyers, who are resisting the subpoena and will litigate it to where they need a judge to decide whether Reddit will have to turn the information over.

That's the process for these things, and we're a couple steps in already.