Their signs are lies.
The people carrying those signs no doubt belong to Mecha. They believe the southwest was stolen from them and they mission is to get it back. Tehy are well on their way in California.
http://www.mayorno.com/WhoIsMecha.html
It's not so much that Los Angeles used to be part of Mexico is a lie. It was part of Mexico. But very few Mexicans actually lived in the states annexed by the USA during the 1840's. There was no air condition or adequate water irrigation, for example. And from whom did those people get Mexico from? The Spaniards, who conquered the Aztecs & Mayas to get it. Those tribes had previously conquered indigenous tribes such as the Olmecs, the Teotihuacanis and others to get it. Is it not a double standard to criticize the USA's claim to it when they got it the same way? And has the USA not done a far better job of making it habitable?