The interesting point lies in how they construct their training data. In addition to producing fake reviews via mechanical turk themselves, they use a complex procedure to retrieve actual positive reviews from tripadvisor.

...which is kind of funny if you think about it: to develop a classifier that can identify real reviews, the first thing they do is create a classifier that can identify real reviews to produce a training corpus.

Then they try to approximate the output of their first (rule-based) classifier with a machine learning classifier.