Sad thing is Grouse, the idiots that abound in this world rule the manufacturers with liability suits. You have to look at things from the manufacturers point of view on this one. If they feel that there could POSSIBLY be a moron out there that would neglect to replace a vent liner before firing then they have to take that into account. To you and those of us who are accustomed to MLers and the possible dangers of NOT replacing such a small thing this may seem like a non issue. But to the lawyers of the manufacturers they look at it as a small feature that would be easily overlooked by a rookie causing damages. In today's world of lawsuits being looked at as a lotto game they have to err on the side of unreasonable to us.