A lot of the hardware on ISM bands is regulated, if only for the assurance that it works on the correct frequency. The key point about ISM is that it doesn't require individual licensing. So I can walk into Tesco and buy a radio transmitter. (Bluetooth ear-thingy).
Changing the aerial of a Wi-Fi unit is very likely illegal. Even though it might reduce the RF Energy going in un-wanted directions. You or I might be able to set up something which doesn't harm other users, but knowing some other users... Can we trust everyone to be the right sort of clever-dick?