Burgers don't ever really bleed, the blood was removed when the animal was slaughtered or it would go bad quickly. When people talk about their meat bleeding it the juice coming out of it, which isn't there if you cook it well done as you've boiled it all off
What do you think is in that juice? Sure the cattle are bled after slaughter, but you never get all the blood out, even after it's been hung, which is still retained in the tissue.
EDIT: Ah ha! My apologies and I stand corrected :) Apparently the red colouring in the steak juice is myoglobin and not in fact blood:
