Clearly, you are not an attorney, and one of those laypeople who knows just enough to be dangerous. Had you any legal expertise, you would know that "threat" includes any attempt to influence conduct in derogation of the subpoena, without actually taking the action, while "force" would include any over act that prevents the witness from testifying, such as removing the feeding tube and killing them.
Greer is not the only one subject to the law. Basically, the marshall can go and take custody of the witness, and anyone who interferes then can be arrested. Like the kids who were arrested today for trying to take water to Terri.
Not a due and proper exercise of congressional inquiry, is that what you are arguing now? Please, give it a rest. I specified, in logical progression, the source for congress' right to subpoena Terri and the other witnesses. If you don't think Congress has the right to conduct review of state health care procedures as part of its legislative responsibilities, or review the due process rights of a citizen under the 14th Amendment, you are way out to lunch.
There was a state judge supporting George Wallace on the schoolhouse steps, too. Federal authority reigned supreme then, because the 14th Amendment trumped the state, and federal authority trumps some pissant state judge when Congress has issued a valid subpoena to a witness in a lawful congressional inquiry. Keep fighting it, you will still be wrong. Now, wrong and stubborn.