I like to use awk instead of grep wherever possible, especially for weird logic like this.

awk '!/B/ || /A.*B/' is one way to skin that cat. If you don't care what order A and B are in on the lines containing both, then awk '!/B/ || (/A/ && /B/)' will work.

grep -E '(^[^B]*$|A)'

EDIT: Whoops, I meant to make this a top-level comment.

EDIT 2: On one client it looked like a nested comment and on this other client it looks top level and now I'm a confused old man.