You should, however, think about the underlying issues. Fact is, the plaintiff proved only $0.99 in actual damages per song from Tenenbaum's use and apparently (I write "apparently" because I have not seen the transcripts, just the report I linked to in the post) chose not to present any evidence of actual damages arising from distribution. It's in the court order, but the actual damages (which is the relevant issue for a 'Campbell' due process analysis) proven were solely from his personal use.
Your "hard to measure" argument is a good one, and the one that the RIAA will raise on appeal. Yet, damages in every case are "hard to measure" and that doesn't stop courts from routinely limiting those damages on constitutional grounds. Can Congress grant copyright holders a presumption in court -- without proving anything at all -- that, for each song made available online for any amount of time in any capacity, the defendant suffered up to $15,000 in actual damages? That's not so clear to me. And you haven't said why it's clear to you.
(I chose "$15,000" because it's the only way the statutory $150,000 could be constitutional under the Campbell case's "single-digit" punitive ratio.)