John,

Because the recirculating ones will not melt down and spread a fire throughout the house like that PVC one will.

It could easily melt down without a fire, causing a restriction for the fan/blower motor, causing it to overheat and catch fire.

Or, a grease flame up on the range itself could set the PVC on fire.

And those are just the first ones which came to my mind.

__________________
Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
www.AskCodeMan.com