If they can trigger a "video fired" message in the analytics, that's one more "Person viewed our video" they can show to a potential advertiser. That's all it's about, not the end user's experience.
A placement that has high first play rate but a terrible 5sec event firing will just be worth nothing. In a programmatic world it means you'll get little to no bids for it or a default placement with payment in the fraction of cents per thousands.
Its usually a deal with a 3rd-party that gives the guaranteed revenue based on views per week/month.