[danner]

That's inaccurate. Operation of unlicensed Part 15 Devices is permitted between 420 and 450 MHz and more specifically there is a sub-band from 433.5-434.5MHz that arduino transmitters use with the caveat that applies to all devices:

(1) This device may not cause harmful interference.

(2) This device must accept any interference received,including interference that may cause undesired operation.