The most-favorable interpretation I can come up with is that Snead believes that justice should always be fair. Everyone should get the same chances; everyone should get the same punishment for the same crime.

There is certainly a powerful slippery slope argument here. The norm that ties well-defined punishments to specific deeds is a strong focal point. In such a situation, it's impossible to move away from this point in carefully measured steps in order to maximize some abstract metric. Once the system has moved away from it, there will be strong pressures to move it even further, motivated by all kinds of strong interests, so it's likely to end up in another (possibly quite awful) focal point, not where that abstract metric says it should be.

(Snead, of course, is assuming much more in the long list from the quoted paragraph.)