Yeah, I'm sure there is a congressional probe because they're allowed to do anything they want. I'm sure Mark Zuckerberg felt that he had to adress this himself because clearly they can do anything they want.
There's a congressional probe because congress is a political body putting on a show, nothing more. Facebook is a private company, they have no legal obligation to be unbiased. They may have business reasons for wanting to appear unbiased, so Zuck is simply managing the company image. Websites are not broadcast stations and don't fall under the same rules as television, calling them a delivery channel is meaningless, there's no such thing in the law.