I know many markets are using food trucks as a way to get more traffic to markets, especially weekday and evening markets. Based on this and other articles that I run across, it sounds like food trucks should be specifically written into market rules to head off this sort of unwelcome publicity.
In New Orleans, we added a “Green Plate Special” many years ago so a restaurant could come and sell for a month of Tuesdays at one of our tents (it was a 10 am-2 pm market then, now it’s 9-1), as long as they had entrees under 10 bucks, sourced from the vendors when possible and followed the specific risk and vendor rules for serving prepared food.

This added amenity was to help us to draw office lunch traffic and it has done that and much more over the years, although I have to admit it killed off a lot of the prepared food items that the other vendors were selling, but maybe that was a blessing in disguise after all. It made those vendors concentrate on their fruits and veg staples and to stop trying to corner the sandwich business at the market.
And even though it was a difficult start (can I tell you the number of restaurants and chefs that I haunted in those early years?) 99% of those that participated over the years that I ran the markets asked to be able to return.

We wrote guidelines for that spot and asked them to pay double what our regular vendors paid which was still a bargain for what they received: shoppers already amassing needing food and meals, in a market with seating and local producers willing to sell items for the menu. So I recommend that markets think about how to include caterers, restaurants and food trucks into their market, but to do it without upsetting the balance of the market too much.
By the way, this article seems to suggest that this is not a “true” farmers market as most of us across the U.S.  define that term, but is more of a food and artisan market. I know Florida has many of those and they seem to be an appropriate market type and serve their shoppers and vendors well in many case but maybe we need a type to describe the market that offers prepared food as its main offering. As I often say to markets when they ask me if a rule is “okay,” it’s only important that the market can defend and explain their rules to their community. If they can, if people around there understand and most agree, then I say full steam ahead.
Okay, one story about the Green Plate. When we developed the idea, we would talk about how we wanted restaurants like Commander’s Palace to do this (often rated as the #1 fine dining restaurant in New Orleans) and although we asked them in the beginning, they quickly sent their regrets (as they are very polite folks). We were seen as a quirky little food event and hadn’t moved to “beloved institution” phase at that point…
After the levee breaks of Katrina 2005, this 100 year old+ restaurant had some damage, needed time to repair and to the great sadness of many New Orleanians, did not reopen that year. However, in 2006, they asked us if they could come to do the GPS, brought their A-team and spread the word that they would sell quarts of their famous turtle soup and a few beloved entrees. So that first day (right after we ring the bell to open the market) we hear a cheer from their tent and see the celebrated chef and owner holding a ten dollar bill over their heads while saying with great emotion, “Our first sale since Katrina!”
The next week, they brought their Maitre d’to manage the line that went out of the market.

So I’ll never forget how our little market helped this great establishment and how our original dream came true all at once. All because we always thought: “what if…”

Anti-Food Truck Meddling Ends Up Ruining Miami Farmer’s Market – Hit & Run : Reason.com.