The rules don't require the person holding the content to scan the content. Normally a DMCA claim is issued before the host has to get involved.
Lets say you and I opened a joint safe deposit box at the bank to share USB drives together with movies on it. If the bank found out, they wouldn't be required to drill it open and scan the drive to make sure none of the movies were copyrighted. This is google drilling open the safe deposit box because someone shared the keys, and they think there may be a movie they don't like there.
>The rules don't require the person holding the content to scan the content. Normally a DMCA claim is issued before the host has to get involved.
Legally, you are 100% correct. The law doesn't require the host to do anything until a DMCA strike is received. But YouTube has its own rules, which is fully legal as well, as long as they are more restrictive than the law (e.g., establishments prohibiting alcohol inside are allowed, even though alcohol is legal; establishments allowing crack cocaine usage inside are not allowed, because crack cocaine is illegal).
And YouTube decided to take proactive approach here. The reasoning for why doesn't matter to the point at hand. The fact of the matter is, YouTube took a stricter approach to DMCA moderation and tends to not always wait until the video receives and actual DMCA strike.
Lets say you and I opened a joint safe deposit box at the bank to share USB drives together with movies on it. If the bank found out, they wouldn't be required to drill it open and scan the drive to make sure none of the movies were copyrighted. This is google drilling open the safe deposit box because someone shared the keys, and they think there may be a movie they don't like there.