> What's wrong with Congress questioning someone about legal but shady activity?

If Congress had invited him to come testify, and he'd accepted, I would have absolutely zero problems with that. But that wasn't the situation he was in -- he was forced to attend a Congressional hearing by federal subpoena, for an action that was obviously not illegal in any way.

At the least, it was a small abuse of power in exchange for a publicity stunt. At the worst, it was unlawful detainment in an attempt to intimidate.