It's revealing that every piece of evidence they demand be shown in the assassination of JFK to persuade them of Oswald's guilt can be provided in the shooting of Tippit. If they complain about the absence of "A" in one case it can be shown to exist in another. Their demands in the second case are met.
The two cases then can be a sort of test or comparison to show the sincerity of a person's demands.
The 'investigating' authorities (your heroes) felt a lot more confident they could make a Tippit charge stick than a JFK charge. Knowing as they did, very early on, that Mr. Oswald had been in the doorway for the motorcade, they couldn't go as far there towards implicating him
definitively as shooter: too big a hostage to fortune. So their shenanigans went all in on circumstantial/'best inference' stuff.
Hence (e.g.) the paraffin tests: 'incriminating' for Tippit; 'inconclusive' for JFK.
