One cool trick to get around this is to cook the beef sous vide. Beef can be pasteurized at as low as 130 F as long as it's held at that temperature for long enough. That's basically medium rare.
After the relevant processing, the argument for cooking burgers to 160 is no different from the argument to cook your lettuce to 160.
If the OP didn't like it because they don't like well done burgers in general then, well, I'm not sure what they were expecting when they ordered one.
What OP actually said:
"It's cooked well-done and can't be ordered any other way."
OP didn't order it that way, Umami Burger won't cook it any other way.
I grind my own beef, and there is no way I would want to eat a burger pattie cooked well done.
If you are worried that your mince may not be safe to eat - why the hell are you buying it?
Properly cooked pork (not ground pork!) should actually be slightly pink in the middle (like with a pork chop). Anything more is overcooked.
/most people have no clue how to cook pork...
It means overcooked to the point the meat is completely gray. Highly undesirable to many (though preferred by some).