I read the answers to "When or how often should I replace my bike helmet?", but unfortunately none of the answers there cites any references.
Have there been any studies quantifying the claim of reduced protection after single impact, e.g. in terms of increased risk of injury?
It's easy to see reasons why manufacturers would want you to believe any impact kills the helmet - it's hard to set limits on potential damage, and buying a new helmet means additional profit.
Horse riding helmets come with similar warnings, but my first one seemed to survive several years while I was learning show jumping - including way more falls than I could have afforded new helmets.