To limit the probability that metal liners will be installed in appliances using fuels that are more likely to cause the accumulation of creosote, which could lead to chimney fires, which could lead to the heat-induced deformation and joint failure of metal flue liners, which could lead to the spillage of combustion products into living space, which could lead to negative effects on the air quality of indoor spaces.
This is to limit the probability of harm to persons.