Bacteria contaminates the exposed surface of foods. Pre-ground meat has a _lot_ of surface area exposed so the potential for contamination is high. If you get your meat as a chunk of chuck roast or larger primal cut, follow good food safety practices in how you maintain your workspace, and then grind the chuck and turn it into patties to be grilled the probability for serving a contaminated product is low.

The problem is that this is more expensive and time consuming so almost no one actually does this in a commercial setting. If you tell me you do this when I come over to your home cook-out I will possibly trust you to serve me a burger with some pink in it, but if your restaurant tries to sell a $6 burger that you let someone order medium-rare bad things are going to eventually happen...