Why wouldn't you strain hamburger grease after cooking?
Well, all I can say is 'it depends on the dish'.  If the dish calls for fat to be added otherwise, you can keep that fat and count it where you would add some later.  If the dish is  just adding hamburger and no more fat - I agree, discard it.

That said - I don't find hamburger fat particularly flavorful and nearly always strain it and add another kind of fat I like better.

It also depends on the fat percentage.  If I'm using lean 97/3 hamburger, there's hardly any fat to drain.