It isn't so much that they should bend over for Sanders so much as it puts them in an impossible/lose-lose situation. That 70% isn't enough to win the general, they'll need some of that 30% too.
Which is why a brokered convention would be the worst possible scenario, and the DNC/candidates should do as much as possible to avoid it. It's bad even if it isn't Sanders who has the plurality. Hard to imagine any outcome of the primary that would dampen enthusiasm/turnout in the general more.