Caching, I'm sure most unfiltered reviews are cached whereas filtered reviews are not and reaching out past the cache can be expensive. One way to mitigate this is to reduce the number of results you pull.
> Why do you need to enter in a captcha to view the unfiltered reviews? Why would they care if you were a bot only for the unfiltered reviews and not the normal reviews?
If you can write a script to deduce the filtering algorithm then you can by definition write reviews that thwart it. With less data, it is harder to deduce the filtering algorithm. In other words, a captcha thwarts high-volume review fraud.
> The filter algorithm seems to be clearly flawed and simply catches way too many reviews that should not be filtered.
I think most people seem to underestimate the difficulty of the problem. Unlike e-mail spam, which is easy for a human to spot, fake reviews are very hard for a human to spot. How can you tell if a consumer was provoked into writing a positive review so that they could get a few bucks off their order just from their writing? You can't, you can look at other statistical trends behind such reviews (such as a sudden wave of positive reviews), but you're only looking for side effects of the primary problem and thus you will never achieve perfect performance from a method like this.
Yelp takes the (somewhat philosophical) viewpoint that customers who are coerced into writing a review are less genuine than they would be otherwise. I believe that this view drives a lot of their algorithm and possibly threatens its accuracy in a way that is ultimately not worth it. I think there are a number of things that Yelp could do to make the users trust in reviews greater that don't involve filtering - one simple thing would be for a user's review of an Indian restaurant to show me that user's breakdown of reviews of other Indian restaurants.
TL;DR: This is a much harder problem than it seems at first glance, partly because of the nature of the problem and partly how Yelp has framed it for themselves.
Disclaimer: I used to work at Yelp, but no longer do. Everyone I worked with were stand-up guys.