When I was in California a couple of months ago, I ordered a hamburger in a restaurant, asking for it to be cooked medium-rare. I was told I couldn't have it that way because they only cooked burgers medium-well.
This wasn't an inexpensive, fast food place but a restaurant, with table service and a wine list. I wasn't being refused a medium-rare burger because the chefs were incapable of cooking one. The reason for the refusal was that the managers were worried I would get food poisoning from undercooked beef and sue them.
Hamburgers have a reputation for causing food-borne illnesses and, every once in a while, they make headlines thanks to mass outbreaks of food poisoning, usually caused by E coli.
E coli thrives in the intestines, rectum and colon of cattle. With beef, the contamination occurs during the butchering process, as a result of poor hygiene and slaughter practices.
A large cut of meat, such as a steak, chop or bris-ket, doesn't usually pose a problem because it's cooked; the contaminated area is on the surface of the beef and the bacteria is killed when subjected to high temperatures.
A hamburger is made up of scraps and off-cuts, so rather than being just on the surface of the meat, E coli can be found inside the patty. Also, the scraps that go into a hamburger are likely to have come from a number of cattle and even if just one of the animals was butchered incorrectly, the E coli in it could contaminate the entire batch. Only by cooking the patty thoroughly, to at least medium-well, can the bacteria be killed.