They aren't guilty of interference. Interference has a strict definition. A device operating in the ISM band by defition can't be interfering with other devices

They aren't guilty of interference. Interference has a strict definition. A device operating in the ISM band by defition can't be interfering with other devices in the ISM band if that bandwidth is being used for communication. ISM (Industry, Science, and Medical) is a free for all band. It is like a public good. Anyone can make a device that uses those frequencies.