This is not so much about not investigating phone hacking per se but a general culture which has been in existence for decades, probably since the start of mass journalism. When I first had professional dealings with press and police this....er...'understanding' between officers and the press in Fleet Street was well established. Nobody gave it a second thought. It was as much a part of life as the police paying informers or hardened criminals being quite good friends with the very men given the job of pursuing them on other days.
Apart from the vice squad operating in Soho, nobody got brought to book for corruption. The reason for that was that the vice squad were taking cash from the criminals specifically for not raiding premises, or arranging to have a tip off so that visits would only find minor porn. That had gone on for years too, but it was such an easy target for the new Commissioner, Robert Mark aided by disaffected or honest constables, and such a blatant suppression of duty in order to facilitate crime, that it could be crushed.
The other stuff, involving the press, had no such stigma. No crime was being concealed, unreported or unsolved in return for bribes paid. The press was just doing its job and the detectives were on a pleasant litle perk , was the belief. And by and large, no big sums were paid as cash or gifts in kind at any one time. It was more a question of each keeping the other side happy all the time, over years, with resultant benefits when particular information was sought or help needed.
What may mark this enquiry out is if particular large sums were provenly paid for specific acts (very stupid), rather than a large sum in total over a period as a well paid understanding.