Under what legal statute, law, or regulation does the US Congress have the right to summon private individuals in order to testify? Doesn’t there have to be a formal investigation underway? I don’t believe, in this case, that there is (although I could be wrong). Otherwise, anyone can be summoned, for any reason at the whim of a dictatorial legislator.
Congressional subpoena power been upheld in McGrain V. Daugherty in 1927.
http://supreme.justia.com/us/273/135/case.html