It's also illegal. Every company is legally obliged to provide the customer with 1 YEAR of warranty on a newly-bought product, regardless of what it is. I've seen sites giving away Xbox 360s and PSPs but those are the ones where you have to get refferals, so I guess they'd send it out as soon as it's released and keep your name on a list. Doesn't interest me in the slightest though, I'm not an Xbox fan.
I would say it's either a joke or this company is out only to scam people since the Platinum 360 pack's MSRP is $399. Also the warranty will be 1 year like the original xbox. As for who would have to cover the warranty I'd say the store would be liable in that case as they are clearly not a MS authorized reseller and therefore the product would actually have no warranty through MS.
Yes, the store would be held responsible for not providing the correct amount of warrenty, but the 1 year is a minimum, as some places decide to give out more, depending on the product, cars usually have the option of buying more warrenty for it, as they break down over longer periods of time.