In simple terms, there are some devices that simply cannot help but generate RF interference when they are being used. So, it was long ago agreed that this RFI is legal, provided that they only radiate on certain frequencies.
The Industrial, Scientific and Medical ("ISM") frequency bands are reserved for the use of such high-power radio frequency (RF) energy radiated as a consequence of industrial, scientific and medical purposes other than radio communications such as amateur radio.
An amateur or other radio communications license is not needed by operators of ISM equipment.
Some of these devices include RF induction heating and welding machines, microwave ovens, and some medical equipment. The RF emissions of these devices --which in some cases can be transmitted very long distances-- can completely disrupt legal radio communications using the same frequency. Therefore, these ISM devices were limited to certain bands of frequencies by international treaties.
Some of the ISM bands overlap amateur radio frequencies and CB channels. Regardless of that, communications operating in these bands must tolerate any interference generated by ISM applications, and users have no regulatory protection from ISM device operation.