The US Government is not suing the people who took other people's material they are prosecuting the people who allegedly stored and disseminated the material.
Under the Courts' interpretation of the First Amendment, in cases of libel it must be demonstrated that publishers' had "knowledge that the information was false" or that it was published "with reckless disregard of whether it was false or not."
I'd suggest basically the same arguments could be made in the defence of MU.
Can it be demonstrated that they knew "illegal" material was being uploaded to their servers and secondly if & when they were alerted to that fact did they remove said material.
Indeed under the DCMA "Online intermediaries who host content that infringes copyright are not liable, so long as they do not know about it and take actions once the infringing content is brought to their attention"
This of course leads into this;
Do you have a link as to who is making those claims?
They sound very much as if they are coming from the DOJ/copyright holders, who of course have a vested interest in portraying MU in the worst possible light.
In particular, given that MU had servers in the US, I find it hard to believe they would fail to comply with a properly executed cease & desist request.
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