The fact that there are other instances when people make subjective decisions is not quite relevant in this instance. We are talking about the case when author

The fact that there are other instances when people make subjective decisions is not quite relevant in this instance. We are talking about the case when authorship on a scientific publication is insufficient evidence that someone had any contribution whatsoever to the paper. So that one needs a recommendation letter, in addition to the authorsip, where the recommendation letter would presumably state, this individual did actually do some work and is not just cruising.