They have the power to call them all to testify UNDER OATH with threat of prosecution for perjury.
But yes, the GOPe are cowards and Darryl Issa is a pussy (shove it feminist).
Issa has subpenaed the IRS Director next week for testimony and hopefully under oath. If it was up to me, I would have subpenaed the IRS IT people first, and then later subpenaed the IRS Director on the same day as the IT people.