It's not the fact that it was manipulative that is generally the problem. It's that something seems transparently manipulative, and hence cheap and artificial and melodramatic. I don't really know how to explain it any better than that. At a certain level, when the way something is trying to manipulate you becomes too obvious, it takes away from your ability to immerse yourself in the work and actually be affected by it.

I mean, yes, all fictional entertainment is inherently manipulative. But part of the point is that we don't want to realize it's manipulative. Otherwise it somewhat loses its function, doesn't it?