No its not. They have a right to protect themselves by removing anything that is deemed illegal. Go read the user agreements. They can scan for anything they don't allow. If you don't want them scanning, you best not be using their service.
According to the iCloud Terms
"You understand that all Content, whether publicly posted or privately transmitted on the Service is the sole responsibility of the person from whom such Content originated. This means that you, and not Apple, are solely responsible for any Content you upload, download, post, email, transmit, store or otherwise make available through your use of the Service. You understand that by using the Service you may encounter Content that you may find offensive, indecent, or objectionable, and that you may expose others to Content that they may find objectionable. Apple does not control the Content posted via the Service, nor does it guarantee the accuracy, integrity or quality of such Content."
Nowhere in the T&C for iCloud does Apple claim any right to access your content and in fact, above they expressly state that they do not control the content in any way.
As for their liability or culpability Section 230 of the Communications Decency Act pretty much relieves online hosting companies, such as Apple, from any liability if someone were to post illegal material in iCloud.
The act also allows providers to take down objectionable material under certain conditions, but nowhere does it give them the right to access your data looking for illegal material. That would be like allowing your landlord or the police unfettered access to your apartment to look for illegal substances. And your landlord doesn't have that right.
Don't you find it ironic that Apple refused to help unlock a phone by a known terrorist but now want's to plough through your data looking for criminal information?