From the "Acceptable use and conduct" section of the Ubuntu One Terms of Service:

All usage of the services must be legal and not infringe any third
party's rights. You must not take any action or use the services in
any way that might bring Canonical into disrepute, affect the ability
of Canonical to provide the services, be illegal, or encourage illegal
activities. You may not use the services in any manner that might be
libellous or defamatory (more info on what that
means), that
contains threats or incites violence towards individuals or entities,
or that violates the privacy or publicity rights of any third party.
Some services have usage limitations as indicated at
https://one.ubuntu.com/plans, and you may not exceed these. As part of
the services, you may store data on Canonical's servers or those of
Canonical's suppliers. You should ensure that this data is not in
breach of any applicable law and does not breach any individual's data
protection or privacy rights. You shall indemnify Canonical in full
for any loss, cost or damages suffered by it as a result of your use
or misuse of the services.

I'm not a lawyer, but I think that makes it pretty clear that Canonical does not allow for Ubuntu One to be potentially used for piracy on a large scale.

At any rate, I don't think Ubuntu One is in any danger as file-sharing is not its primary purpose: synchronization between your own personal machines is. I'm not a user of file-sharing sites like Megaupload, but in their case (from what I understand) their site was primarily meant for sharing vast quantities of files between anonymous users. If Canonical's primary plan for Ubuntu One was to facilitate piracy, then they would be more likely to become a target.