It's probably false that "there does not exist a customer of EC2 that has been hacked", and that should be pretty obvious. Furthermore it's false for essentially all nontrivial, large services.
In other words, better safe than sorry.
If the compromise was due to a flaw in Linode's system (potentially exposing other accounts) then a global password reset makes sense.
Can you imagine if every service you used reset everyones passwords every time one of their users got brute forced? You'd do nothing but reset your passwords all day...