they are rubber i believe. i think some of you guys are a bit confused, the chassis is the actual inside part of the laptop, the frame on which the rest is built. that is an alloy i believe. the same was the case for the area 51m 5500 that AW used to make. it had an alloy frame with an outside case of plastic.
the lack of reinforcement is kinda disheartening but that happens, likely a compromise was made between weight and size as well as the costs. sad but thats kinda how it goes. the plastic case if plastic but carbon fibre cases are not perfect either, as strong as carbon fiber is it has this ability to shatter when hit in certain ways too.