Net Neutrality, Common Carriers and Legal Liability

(For background on the issue of Net Neutrality go here).

I have a question (warning: this is based on my layman's understanding of the law)

Net Neutrality, the principle that internet service providers cannot distinguish between the originators of content (or the nature of the content for that matter) when giving priority to traffic on their networks, makes said service providers the internet's version of a Common Carrier. The service provider provides a common method for the transportation of content from providers to consumers. The law with respect to Common Carriers says that said Carriers cannot be held legally liable for the content they transport as long as they treat it as equal. For example, a service provider cannot be sued for libel just because their network was the vehicle by which a libelous article was transmitted.

But, if a service provider gives preference to one type of content over another, are they not vesting their will in the content they are giving a preference to? If some content provider pays the service provider money to give their content preferential treatment on the provider's network, doesn't that mean that the provider takes on a measure of responsibility for the content itself?

For example, if AT&T signs an agreement with MSNBC to give video traffic from the latter's channel priority over traffic from CNN and FOX, and MSNBC transmits a video that contains a libelous attack, doesn't AT&T become liable in any subsequent legal action?

Here's my point: Do the service providers appreciate what they might be getting themselves into by backing legislation that would eliminate Network Neutrality? Do they really want to open up that Pandora's Box? And couldn't defenders of Network Neutrality use this as an argument to persuade service providers that they are working against their best interests?

Just a thought.