Because Youtube presents itself as being agnostic to the content and letting anyone upload anything (excluding porn and dark stuff that any reasonable person would deem inappropriate like ISIS propaganda or child abuse).
If Youtube presented itself as being a video platform for <insert political affiliation> from the getgo, then I wouldn't see a legal problem with moderating videos containing opposing views.
If Youtube presented itself as being a video platform for <insert political affiliation> from the getgo, then I wouldn't see a legal problem with moderating videos containing opposing views.