Why wait 20 years? Um... I'm thinking it has something to do with $$$. But the plaintiffs, Knack's front man Doug Fieger and lead guitarist Berton Averre, swear they never even heard the Run DMC rip-off until August 2005. This blogger wishes she could say the same about "My Sharona" :) It could also have something to do with Fieger's recent surgery to have two brain tumors removed... when you're reminded of your own mortality like that, it tends to make you wonder who you could possibly sue before you die.
But seriously, what has Apple or Amazon or anyone but Run DMC done wrong? These companies barely existed back then, if at all, and they certainly weren't in the music business. Why should they be on the hook for selling music that no one raised any eyebrows about for twenty years? Of course I, being an arrogant, elitist music snob, might say they all get what they deserve for doing anything at all to facilitate the distribution of Run DMC or The Knack, but I suspect that is not a valid legal argument.