Radio receivers are very sensitive and you're very close to your computer, both factors that make shielding difficult. And, the shielding isn't perfect -- ther

Radio receivers are very sensitive and you're very close to your computer, both factors that make shielding difficult. And, the shielding isn't perfect -- there are holes for fans, some cases are made out of glass instead of metal, etc. There are regulations, but they don't mandate perfect attenuation. They only specify that your emissions shouldn't interfere with licensed users. Since the licensed users are pretty far away from your computer and the power is very low, it's generally not a problem. (As for sensitivity, remember that GPS signals have a power of something like 10^-16 watts at the surface of the earth, and you can receive those just fine. Radios are very sensitive instruments!)