ive seen much smaller switches that can take 10A @ 120v, i dont think current is an issue. i think they just needed something to do with them so they figured "eh, lets just connect it a bunch of times"
I agree it probably does have to do with the amperage. Manufactures are likely to choose the cheapest parts they can source. While there are much smaller switches than can handle much higher amperages just fine, there are even larger switches than can handle even less. It has to do with the quality of the materials used to make the switch.
Its quite possible that with the materials used, the switch could handle 2A just fine if it were a DPST, but then manufacturers are always conservative with the specs of the parts they make, in order to minimize returns. Nintendo is likely to just follow the specs rather then test the rating of the switches themselves.