There are often explicit permits for street vending of food along with inspection requirements and the permits often do rise high. Where there aren't permits there usually isn't the density to support it in the area chronically. Sanitation limits what can effectively be served without a sink and promotes disposability. There used to be ice cream stands about a century ago which had deposits on glass but well, several epidemics gave a very valid reason for the ban as popular as it was in hot summers.

Regardless there are always complaints about food standa or trucks anywhere from traffic to matters as petty as park service trashcan fill rate.