History

The issue isn't so much a moral one (Molotov's proposed "from each according to the quota, to each according to the wage-scale" is if anything less friendly sounding). The issue is a theoretical / honesty one: both clauses of "from each according to their ability, to each according to their work" aren't true under socialism, under the lower stage of communism.

"From each according to their ability" is impossible because e.g. shitty work conditions and shitty wages mean workers won't give it their 100%. "To each according to their work" is demonstrated false when e.g. idlers and hardworkers are paid equal amounts. To quote Molotov:

It's nothing but window dressing. But window dressing is intolerable in Marxism. Marxism is an objective science; it views things soberly. It calls bad things bad and good things good. It demands genuine, uncompromising struggle for the good. Window dressing is not allowed. ... Here we see young people growing up; honestly they say: this is stupid. What our elders babbled to us does not correspond to reality.

This doesn't apply to communists, to revolutionaries. Molotov says we should "destroy what is bad and sacrifice ourselves if necessary" without regard to pay or conditions.