Fundamentally in Los Angeles, it is illegal to shoot a film in a public area without securing a permit. Especially if you would be obstructing the normal traffic or pedestrian flow. Thus, Los Angelinos expect the notices. If you don't have one of these, there is nothing you can do when some asshole decides to take a shortcut straight through your shoot.
But permits take time away from your shoot, they cost money, and sometimes you find a locale you love, but you can't secure the location through a permit. What are you gonna do? Go guerrilla, of course! Guerrilla filmmaking tends to happen quite often in indie and student films. I did it myself. And despite the clearly laid-out rules at USC about shooting without permits, the school itself turned a blind eye to it in regards to its students. There was even a class at 'SC about Guerrilla filmmaking. Guerrilla filmmaking only works if you have a small cast and crew and very few equipment, or if you are shooting a documentary. That way you can shoot and move more quickly.
If you are caught by authorities, however, you could face legal issues. Big legal issues. And who wants that when they are trying to make something meaningful to humankind? The compromise is to secure a permit with a small crew. Here in LA, if your crew is small enough and you have no stunts or SFX, your permit may be free. Small crews are favored over large ones, and you can control the locale to your liking. In other cities, you may even get the support of the local chamber of commerce or film commission, and even some soft financing (that's price cuts and in-kind donations). Sometimes it's worth following the rules. For a list of film commisions, click here.