I broadly agree with what's been said here, and wouldn't favor such a regulation at least as described here. Some thoughts on the matter:
1. Density and space in between seats is absolutely a valid area to regulate. The "distortions" are often the whole point of regulation, and this is no exception: if a market free from distortion produces cheap fares that people want to buy, but unsafe planes, we want some distortion!
2. However, I feel current regulations adequately address this issue. They already make airlines run evacuation tests on their proposed layouts. If they squeeze the seat pitch to unsafe levels, it should show up there.
3. Regulating solely for passenger comfort seems like a poor idea to me. Let there be market segmentation between cheap and uncomfortable Spirit flights and still pretty cheap and slightly less uncomfortable mainline carriers. The industry has done really well since the original deregulation of fares and such, so I say let's run with that approach.
4. Another drawback to regulating pitch directly is that pitch isn't necessarily a perfect indicator of space. Different seats at the same pitch can have significantly different amounts of legroom. Generally this is associated with new "slimline" seats, which people around here like to gripe about for lack of cushioning. But suppose somebody truly innovated with something like suspension fabric (think Aeron desk chair) that was nice and comfortable and had decent legroom at 27" pitch; wouldn't that be a huge net win? Do we really want to discourage that?