Not condoning what they did, but saying nothing seems to be par for the course, based on limited personal experience, and a lot of chatter from colleagues the past few years. Sad, but true.
I wonder if any companies know/care how much this kind of behavior negatively impacts people's perception of them.
One thing I've heard from larger companies is they often do not say anything because almost anything can be construed in to a lawsuit. I think this is a bit of a copout, and I think in most cases, it's just easier to do nothing, but the lawsuit thing probably isn't a total non-issue for some companies either.
"Thank you for your time and interest. We were faced with a tough choice with a lot of candidates, and we ultimately went with another candidate for this position" seems as inoffensive and bland as could be - why these aren't sent out it, I don't know.
As with any startup, we are a small team doing a million things, and sometimes things (such as interview rounds) may take longer than they should because a critical need for the business was prioritized.