I'm guessing they feel (probably with good reason) that Nintendo would stop sending their store Wii's if they sold them for $300+, but probably don't care much if they tack $5 on an accessory or game (as Gamestop does with a lot of DS games etc.).
Nintendo probably wouldn't like the negative PR of stores selling the Wii for above MSRP. It going high on eBay is fine, as it shows high demand, hoarding etc. But stores like Best Buy selling it for more would probably be viewed pretty negatively in general. More on the stores, but probably raise some ill will toward Nintendo as well (eventhough it would be unwarranted).