FWIW, the subpoena may be for his testimony in a case unrelated MTG's operations directly. Also, I think there must be a pending suit in a civil matter or charges filed in a criminal matter before a subpoena can be issued (the suit or charges are against someone one else, you don't get subpoenaed to testify in a case in which you're already a party). His options are to quash the subpoena or take a free trip to where ever he has to go and give testimony/deliver documents in lieu of a trip. Don't read too much into this.

Also, for your reading pleasure: http://www.law.cornell.edu/uscode/text/28/1783