One Mouthy Broad

Part of it's a feedback loop, I think - companies seek to cover their backsides from every potential eventuality, and thus warn that their product may contain nuts - not bothering to mention that it's as likely as the sun failing to rise tomorrow. Sticking a warning on costs nothing... except for the part where it panicks already stressed parents worried that their kids might be in danger, and who now fear that the dreaded toxin may be lurking anywhere. Panicking parents make companies more concerned about making sure they won't be held liable, etc.