Why do we lose LP for games where a teammate is subsequently banned for griefing/inting/AFKing?

No one knows.

Becuase the flip side is you get zero lp for winning vs some one who does that behavior.

Because there can never be a reward - even an indirect one - for unacceptable behavior. If there were, that would encourage such behavior. For example, a duo booster might be willing to sacrifice their smurf's Leaverbuster/etc. status whenever it looks like their client might lose LP. Riot is trying to reduce rule-breaking behavior, not encourage it.