A better explanation would be that YouTube gave UMG (and other 'content' companies) a kill switch that works on any video and said 'Promise you'll only use this

A better explanation would be that YouTube gave UMG (and other 'content' companies) a kill switch that works on any video and said 'Promise you'll only use this on stuff that infringes your copyright, ok?' UMG then crossed their fingers behind their back and said 'Sure, whatever.' The second party of your comment is correct. SOPA would give these same companies a kill switch for websites instead of YouTube videos.