You still have to pay for campsites. The free pass is to allow you to stop, hike, etc. within park boundaries.
This from the PC Website about them being overloaded with traffic:
"Parks Canada is thrilled that so many Canadians are excited for Canada150 and the free souvenir 2017 Parks Canada Discovery Pass. Due to the exceptionally high volume of Discovery Pass online orders, you may experience delays in accessing the site.
We wish to remind those who are trying to order their pass online that this is not a limited time offer and encourage you to come back and try again later."
