Could be for several reasons :
1) some such units require metal pipes for first 12 or 24 inches from the unit for heat protection - either by manufacturer instructions or by local building code requirements,
2) since some such units require it by manufacturer installation instructions, he may have seen it on other units so just did it on yours regardless - most installers I have seen do not bother to read water heater or boiler/furnace installation instructions.
3) This is a catch 22 which most people, even most plumbers, do not know. Generally, plastic piping manufacturers say, buried in their product info that no one ever reads, that plastic pipe should not be used within 6 or 12 inches of the surface of a fired device (boiler, water heater, etc) or with 6 inches of the opening of a draft hood - the open-bottom cone on the top of the heateer where the flue goes into the exhaust flue ducting. Therefore, even if the heater manufacturer does not require it, the piping manufacturer may effectively be saying you need a 12 inch metal nipple before you go to plastic. This may or may not be in your local area's plumbing code. I know you can puy 12" stainless nipples for your situation where hard water causes buildup in the nipple - which because it is the only metal pipe in the vicinity, will happen much quicker at that nipple than if the overall piping system is all metal, because electrolysis is occurring at a specific point rather than spread out. Personally, I would never use plastic pipe within a couple of feet of a draft hood - I have seen several cases where a flare or backdraft caused nearby plastic pipe to catch fire in that sort of situation, and once plastic pipe gets going you have a REAL problem because it burns fast, hot, and incredibly smoky. Why take the chance. Personally, if one was going to use plastic pipe there, I would have a substantial vertical run from the heater if possible (a couple of feet), and slip a foot long piece of metal pipe a couple of sizes larger over it as a flame protector sleeve - making sure it does not contact dissimilar metals anywhere.
