malaysiakini has this feature asking the question:
Why doesn't Pak Lah fight back?
It seems that the PM is prepared to accept many blows from TDM and only occassionally puts up a defensive block when the punishment is too unbearable.
Following the March elections I reckon most UMNO leaders especially the PM realises that the BN parties need to undergo major makeovers before they can hope to do well in any further elections and therefore having a public confrontation would only damage the party further and possible put it beyond redemption.
Furthermore the PM cannot be seen to be a vengeful and spiteful person as he needs to uphold the dignity of his office. So he will only respond by granting the AG's office more competent staff and better resources to pursue the work created by the Lingam Commission.