Why did the CIA's Angleton want to cut off questions about Oswald?

In response to my recent post on a declassified April 1972 CIA memo ordering that "no defector or source" be asked about Lee Harvey Oswald, a faithful reader asks:

Where is April 1972 in the Nosenko chronology? Was there a time at which saner CIA people simply told Angleton to back off from his Nosenko-KGB theories?

The answer is that Angleton was motiva…