You could never trust that the attacker actually deleted their copy of the repo, but then, the whole cryptolocking business model falls down if the attacker isn't at least moderately honest, so I can see why people would respond to that threat.
Nitpick: it only requires most attackers to be somewhat honest. Having a few unscrupulous ones may make life harder for the “honest” ones, but they themselves can be better of, e.g. by, after receiving payment, demanding more money.
In the same way that it's worse to shoot someone with an actual gun than to threaten to shoot them with a Nerf gun.
The negative network effects on other scammers are also nice.
An "honest cryptolocker" helps support more cryptolocker use, as people trust that if they pay the criminal they'll get their stuff
If dishonest ones were the norm, than maybe cryptolocking would cannibalize itself as nobody would pay since they know its useless. So in a sense the dishonest one while having less ethical intention has more ethical results. But only at scale. Hmmm.
This is a classic prisoners' dilemma: if no one payed, every one would be better off, but it is very hard to be that one guy or company who loses all his files for the "greater good".
society works with mutual cooperation and hackers seem to understand that more than the "technically cooperating in this context" that the legal field would employ
(This is probably not true, but society would benefit from "cryptolockers are usually fake" being in the zeitgeist)