That is an assumption. None of us are mind-readers, thus none of us can know for certain what the reasoning is behind a like/rating. For example, I'm very generous with likes. My reasoning is that if I like, agree with, appreciate or think a comment is useful, it gets a like.
Putting limits on positivity is the same as being negative. Limits on the downrating are needed as they are the most likely aspect to be abused.
Can't agree with this either. Many times I use likes to acknowledge what someone has said and let them know such, which is why the current like system is valuable as it shows not only that someone agrees, but also who. In those instances, adding a comment without much more to say is not needed.