It's a strange phenomena, and I'm not entirely convinced that it doesn't imply poor quality entertainment. Good quality entertainment is eminently re-watchable. How many times have I seen "Groundhog Day", or read "Lord of the Rings"? The fact that I know exactly what happens doesn't seem to diminish my enjoyment at all. In fact, it seems like with every re-experience I pick up some new detail or insight.

OTOH I can totally understand spoiler alerts with respect to sport and sporting-type events (like politics). It usually does seem to diminish the pleasure of watching a game when you know the outcome.