The L.A. Times says the move is driven by pure greed on the part of brick-and-mortar restaurants, and that the new legislation attacks an L.A. institution:

Forget the Getty — it's the taco trucks, and their crowds, that are the true culture of L.A. Attacking the trucks is like New York going after its hot dog stands or Memphis banning barbecue pits.
 
And other than raw greed, I can't see any reason for it. Ron Mukai, an East L.A. developer, says the trucks are unfair competition, edging out the "legitimate brick-and-mortar businesses." But the county's 14,000 registered catering trucks seem just as legitimate as restaurants—they're just providing a different service. Restaurants provide meals, and a table to eat them at, and walls to eat them within. Taco trucks provide food, pure and simple. They charge less because they're selling less.

These things aren't cheap or makeshift, either.

Shortly before Thanksgiving, a furtive crew of workers for L.A. Outdoor Advertising poured a cement foundation next to the Harbor Freeway and anchored a huge metal structure into the wet cement. A few days and roughly $100,000 later, the crew had erected L.A.'s latest illegal billboard atop an equally illegal 10-ton superstructure that can be removed only with a wrecker.

for its crime, L.A. Outdoor was "cited" and "ordered" to take down the illegal billboard "immediately." Five months later, that billboard still looms large. City Hall has caved to outdoor advertisers for so many years that L.A. Outdoor is touting the illegal billboard in a photo array on its Web site — a bleak reminder that billboards run amuck here, and their owners enjoy impunity.

Clear Channel Outdoor, CBS Outdoor, Vista and others use the legal system as a delaying tactic, filing lawsuit upon lawsuit. City officials so badly fear the wrath of the billboard companies that they resisted giving L.A. Weekly basic, public facts about existing legal and illegal billboards. Plenty of U.S. cities have required the firms to hand over their inventory lists — a necessary step before activists, neighbors and inspectors can ID and dispute illegal billboards. Houston forced its billboard companies to hand over a list. So did Philadelphia and San Francisco. Florida''s Department of Transportation obtained its list — in 1972.

But in Los Angeles, the newspaper had to hire a First Amendment attorney to obtain simple information from quaking workers at the Los Angeles Department of Building and Safety, a taxpayer-funded agency that deals almost exclusively in public data. For months, department spokesman Robert Steinbach refused to talk, behaving as if he were protecting the national security.

"Billboards Gone Wild: 4,000 Illegal Billboards Choke L.A.'s Neighborhoods" [LA Weekly] (Thanks to Elliott!)
(Photo: r_neches)