//They do not issue subpoenas in order to interfere with court orders.//
Congress wants to see whether Michael is telling the truth about Terri's condition. Michael's behavior provides substantial cause to believe he is outright lying about it. Michael claims she's entirely incapable of communication, but the parents claim differently. Calling her to testify would clarify things a lot, especially if she was tested to ensure Michael hadn't drugged her.
That is not Congress' purview. They are a legislative body. They can only call people to testify on pending legislation, not to determine whether or not they have lied to courts.