http://www.canadapost.ca/cpo/mc/business/productsservices/atoz/servicelabel.jsf#Return
Got that off Google.
I think the basis is that all mail tendered to Canada Post must have postage paid. An opened package that is resealed becomes "mail", because the policy in the link clearly comprehends that "return to Sender" will only occur in cases where mail is undeliverable.
But you can use the "Contact us" link and ask them about it if you want more details.
Of course, you might wind up on another "list" for doing that, but that's the risk.]:)
Imagination will often carry us to worlds that never were. But without it we go nowhere. Carl Sagan