In Canada, it's outright illegal to sell a burger medium done. If you get caught, your restaurant will be closed for a couple weeks minimum, along with a hefty fine. It must be fully cooked, or not served.
If they grind their own meat or use something like ground chuck or ground round you can get it medium.
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...
Some meat sellers mechanically tenderize (blading) their cuts of beef. This may cause E.coli on the surface of the beef to enter the interior of the beef. Costco was linked to E.coli cases in 2012 due to this practice. See https://www.consumerreports.org/cro/magazine/2013/06/has-you...
If you can serve a steak medium, then you can also grind the steak, cook it medium and serve it.
Only two reasons to ever cook beef: (1) taste (2) quality