Strict prohibition on overlapping data. Why?
I think you're reading a little bit more into the policy than is actually there - many papers in this type of journal essentially contains data - gels, lab results, etc. That's what they're talking about. What they're not talking about is the corpus of data generated from a whole study.


  Why not?


Because the goal is not to allow researchers to publish repeated analyses of the same dataset.


  What if the two papers analyze rich sources of data that overlap only
  partially?


It's likely not what they're talking about - a rich source of data likely means that the data hasn't been published recently. What they don't want is "The same data as last time, now with a marginally different story".


  What if they have completely different ambitions?


I'd be impressed if you managed to get two completely distinct analyses out of the same data - and indeed, the stance of the journal would likely be that they'd be willing to accept a decrease in salami slicing of papers at the cost of the occasional genuinely novel analysis that's built entirely off previously published data.


  Is the only meritorious activity in science the act of gathering data,
  and to hell with the analysis?


That's reading way too far into the policy - the paper should also contain that analysis. What they don't want you doing is writing four papers on the same single assay or whatever.


  And what does this have to do with ethics?


The idea that a paper should represent a single, stand-alone contribution to science, and that ever more esoteric spins on the same data is a net negative is an ethics in science stance. One you may disagree with, but it is one.