While all of that is true, there's an additional
thing going on there...
Toner has an affinity for adhering to certain materials - one is the vinyl
that 3-ring binders are clad with, the other is toner itself.
It takes much longer for double-sided copies to stick to themselves, but
under a variety of storage conditions, it does happen. Sticking to binder
covers is much quicker, and as someone else here pointed out, ensuring there
is a blank page at the front and back goes a long way to preventing problems.
Rymans (a major UK stationery shop chain) sell cardboard-cover A4
binders. I've never had photocopied/laser-printed sheets stick to those.
-tony