What do you mean there's no proof? Hundreds of thousands of users can attest to the illegal fire sharing Megaupload was famous for. Now I'll have to ask you if any jury will find it reasonable that the execs at Megaupload didn't know about this illegal file sharing?
Furthermore, the entire business structure of Megaupload was based upon illegal sharing. Users only needed to pay if they wanted to download a lot. Now what type of legal files uploaded from random unknown users, would get that much attention and attract people to pay for bandwidth? Files found only on Megaupload, not on the free photograph sites or Facebook?
Any intelligent unbiased person would have to conclude that the business structure was based upon illegal file sharing, the business model would not work otherwise. As for corporations using megaupload, give me a break, why would you want confidential files or private information to be "shared" and available to anyone who gets their hands on the link? No way, that's a lawsuit and liability waiting to be filed. If you have unimportant documents, then there's Google Docs.
Speaking of Google Docs, there's a legitimate site that doesn't depend upon illegal file sharing. Notice how it's free, why would anyone bother with Megaupload unless it offered illegal files? Again, it doesn't take a genius to figure it out and a jury will convict.