As someone with quite a bit of Yelp experience, the filter doesn't just filter out people with a single bad review, it also looks at the distribution of the sum of all the reviews on that account. Generally, over time with enough reviews, each user generally falls into a similar pattern systemwide with a pretty regular curve of rating scores distributed over the reviews. Anything deviating from that curve more than 'x' amount gets filtered (it's secret so you can't game it). It's pretty pronounced and predictable - so the sourpusses that leave nothing but bad reviews get filtered no matter how many they write. Same with the people that leave nothing but glowing reviews.
In my personal experience, the small businesses claiming that Yelp or a competitor are targeting them with bad reviews are full of it. I just go look up their BBB score and almost always see the same types of complaints against the business there. There generally is agreement between a trip advisor rating and a yelp score as well. Sometimes its hard for people to look at their operation and realize they truly do suck. You see it all the time on those reality shows called "Kitchen Disasters" or "Save my Restaurant" with that foreign chap from Hell's Kitchen. They always think they are rock stars and have no idea why their business is failing when dude shows up.
I have yet to come across a business with multiple well-written (a couple of paragraphs with concrete examples) bad reviews that were legitimately attacks and falsehoods made up by competitors. Granted it's possible, but in my multiple years as a yelper "elite" and with the ~500 or so reviews I've written, I haven't seen it. When people take the time to leave lengthy negative reviews, they are usually legit.