Presuming someone to be lying is a huge presumption. Innocent until proven guilty should be a rule used in all cases (in the case of a 'lying' police officer, the charge is perjury).
If they can prove neglect of the evidence or deliberate tampering, of course, that would be another story.
Eh it's not about perjury, it's about due process and assymetry of power. The police have the means to record everything at little cost and with no reasonable excuse to not do it. The absence of recordings is a reasonable indicator that the cops are hiding things.
We expect the state to follow processes strictly. The penalty for failure to do so isn't personal incarceration for the prosecutors or police, but rather failure of their case. This seems like a tradeoff that works in practice.
i remember being on a jury in a case that involved child abduction, the police did not record the interviews they conducted with the child after it had been rescued. i talked to the prosecutor afterwards about it and they said it's to easy over analyze every word said and twist it to appear as something else. to easy for a defence to say they were leading the witness and therefore the recording should be thrown out. remember there was video of the Rodney King beating. the defendants slowed it down and pointed out on each frame actions that King supposedly did that were threatening to the officers beating him.
recording everything is not a panacea. not having everything recorded a sign of nefarious actions.
This amounts to the "you can't handle the truth!" argument. It may be true that the average citizen, and thus the average jury member, is unaware of what ethically-conducted police work looks like. But that doesn't seem to constitute an argument that keeping everyone in the dark about what average police work looks like is an overall good thing.
If they can prove neglect of the evidence or deliberate tampering, of course, that would be another story.