Most bay area high schools I’m familiar with only check for completion for hw, not just math, science as well. There are a number of reasons - one is that teachers don’t have time to check all hw submitted for correctness, second it gives the student flexibility in how much time to spend on hw without the stress. Another reason is that there are kids that don’t need the hw to understand the concept, so making them do it seems a little counterproductive. The OP is correct in that h/w or problem sets are there to give a slight boost in the grades, so the B- would probably have been a B.