I dunno, my snarky answer would probably be: Just as if they'd test for A, but you actually need to be good at B.
I've not been to any school in the US and I don't have a PhD, so all these acronyms like GPA only have a vague meaning to me. But as someone who kinda disliked school and didn't put much effort in it, I can see how school grades can be a bad proxy for university. I didn't even do that well in math in school, but I managed to go above and beyond with all my math classes for my CS degree. I'm not completely sure how the admission for PhDs work and if they look at your BSc/MSc tests... but again, I guess doing ANY work related to finishing a paper is probably closer to what you'll be doing later than e.g. your grades in database stuff when you'll be researching programming languages...
I've not been to any school in the US and I don't have a PhD, so all these acronyms like GPA only have a vague meaning to me. But as someone who kinda disliked school and didn't put much effort in it, I can see how school grades can be a bad proxy for university. I didn't even do that well in math in school, but I managed to go above and beyond with all my math classes for my CS degree. I'm not completely sure how the admission for PhDs work and if they look at your BSc/MSc tests... but again, I guess doing ANY work related to finishing a paper is probably closer to what you'll be doing later than e.g. your grades in database stuff when you'll be researching programming languages...