Condiments are still better than acid, flammable liquids, lit cigarettes and matches, fireworks and other things that have been shoved down bookdrops. That's the main reason bookdrops are, whenever possible, separate entities out by the curb instead of being through-the-wall slots -- which in new construction require a separate sprinkler head! The modern curbside bookdrops will snuff a fire within seconds (lack of oxygen) and limit the amount of damage to the returned materials. They will also withstand the explosion of an M-100 firecracker, equivalent to approximately 1/4 stick of commercial dynamite. And I'd rather folks had to hand mayonnaise-covered materials than stuff covered with battery acid.

Oh, I find I forgot to mention those wonderful people who use glue for various tricks....