The idea is that if an unattended bag is found, it can be deemed to not be 'suspicious' and returned to it's owner.
Judging by one of mine that got wet, they are not just biodegradable, they seem to dissolve.
Surely anyone wishing to place a suspicious bag could easily get hold of a label. If they can buy dangerous items without being stopped then getting hold of bits of pink paper is a doddle.