I think the main point is that there's a difference between slackers and people who don't care. You can slack but still care about the group work, but you can a

I think the main point is that there's a difference between slackers and people who don't care. You can slack but still care about the group work, but you can also slack and not do anything at all. In the former, slackers may feel compelled to work harder when working in a group (like the ones you're talking about), but in the latter they just don't care enough (like the ones who ignore emails).