In the early days of link aggregators there was a strong implicit rule in most communities that the downvote button should NEVER be used for disagreement - only to mark responses that a user felt did not contribute to the overall conversation.

Nowadays people hit downvote on anything they don't agree with. People don't like seeing a large count of people who clicked the "You're wrong" button on what they said.

I don't know why reddit thought that people would use it any other way, they were basically asking people not to use it how the vast majority would presume it should be used.

The reddiquette was strongly enforced by the community for the first several years. Eventually reddit got Eternal Septembered and it was just too many newbies for the experienced ones to police.