They probably feel it derives from their power to regulate interstate commerce.
Also, Congress is not prosecuting anything in these hearings, just trying to gather information about what happened and hopefully rope someone into saying something that gives evidence for a prosecution.
Now that isn't saying that they aren't PERSECUTING these executives, merely that they aren't prosecuting them.
Frankly, I think the executives in the Enron, and Worldcom cases need to be closely questioned and prosecuted if wrongdoing is discovered. What their companies have done is defrauded their investors, customers, employees, creditors, etc. out of their hard earned money in order to make profits for major stockholders and board members at the very top of the food chain.
Now I have no problem with the folks at the top of the food chain making as much money as they want off their investments, so long as they do it honestly and ethically. To me that doesn't mean stripping the company down to bare bones to line your own pockets, then selling off the husk and leaving teh workforce and customers out to dry. That's not ethical behavior.