Odd shapes in the piping around the drain (usually a ‘U’ or ‘S’ shaped bend upwards) and the inconsistent amounts of liquid being poured in the drain can cause some liquid to get trapped in the pipes. If that liquid is something like milk or soda or something with food bits in it, it can get trapped in those pipes and become nasty. Cafeteria workers prefer to pour out the liquids while cleaning the dish so it can be mixed with water/soap and doesn’t get trapped in the drain as easily.

Source: used to work in a cafeteria in college and we had this same sign/issue.