Because it will not draw more than it needs and it cannot draw more than the output is. A 1 Amp wall wart would simply be working at less than its' maximum, which is usually preferred.
Oh - I thought it was implying you needed a 1 amp wall wart and that didn't make sense to me w/ respect to a USB port's specs. Seems if it works with a USB port a wall wart would only have to be 500 mAh not 1 amp. A lot of the 'warts' are actually less than 500 mAh, I guess that's why they recommend a 1 amp one.