A subpoena is a signed litigation document, which means it's either ordered by the court or issued by an attorney of record. Abuse of the subpoena can result in sanctions on both the attorney who signed it and his or her client. It can also run into various statutes (e.g. the 9th circuit allowed a suit to proceed against an attorney who issued an overbroad subpoena for e-mail under the CFAA under the theory it was equivalent to hacking the e-mail provider). Extortion is of course also illegal. Finally, witnesses can invoke the 5th to avoid producing information that might incriminate them in a subsequent criminal case.

As a general rule, there's "no money" in doing something like that. You run the risk of getting sanctioned, and if you use the information improperly in other litigation, it'll just get excluded as having been obtained via abuse of a subpoena.

The bigger worry is stuff like this, which doesn't happen under court supervision: http://www.wired.com/threatlevel/2012/08/administrative-subp....