This is because 1) the responses retrieved by retrieval-based method are actually written by human, so they do not suffer from grammatical and fluency problems, and 2) the combination of various feature functions potentially makes sure the picked responses are semantically relevant to test posts.
it is a bit surprising that this can be achieved to a reasonable level with a linear transformation in the "space of representation", as validated in Section 5.3, where we show that one post can actually invoke many different responses from NRM.