Downvoting has a negative psychological effect, that´s why social media tends to avoid it.
Allowing people to interact negatively with ads (in the sense of "paid messages") in a way that affects public perception of the message reduces the value of purchasing advertisement; since FB uses a model where essentially content of all types can be ads (that is, paid messages) -- since "free" messages of any type of limited reach which you can pay to increase without changing the type of message -- allowing negative interactions would interfere with their revenue modelb reducing the value of paying for reach.
And that's why there is no Dislike button on FB. You can choose not to see things, and you can interact with things that make other people less likely to see them. But you can't choose absolutely to get things in your feed, and you can't interact with them in a way that reduces other people's likelihood of seeing them to less than it would be if you didn't interact with them at all.