It’s a real problem. Just because a device is not labeled as a transmitter, does not mean that it doesn’t transmit, and just because it it is only supposed to transmit on a specific frequency, does not mean that it doesn’t transmit on other frequencies. You have to make a distinction between ideal behavior, only found in text books, and the real world. Almost anything that uses electricity is a potential source of interference.
Radio and television receivers can produce interference via local oscillator radiation. FM broadcast band receivers are well known for being a potential source of interference to systems that use the VHF aeronautical band, like navigation beacon receivers and voice communications receivers.
Almost all transmitters have spurious outputs, which can be suppressed (attenuated), but not eliminated. They also produce broadband noise. These can interfere with systems that handle weak signals, like GPS.
Anything with digital circuits, which includes almost everything these days, is a potential source of interference. Digital circuits can generate large amounts of RF noise over a wide range of frequencies.
Even things like simple amplifiers and poor electrical connections can produce interference via intermodulation distortion. If you have several nearby transmitters with strong signals, any non-linear device can behave like an RF mixer, producing new signals that are related to the sum and difference of the input signals.
Audio amplifiers can detect and amplify the AM components of signals like those produced by GSM cell phones. That’s why you often hear funny noises when a GSM cell phone is near an audio amplifier. The amplifier is behaving like an AM radio receiver. The unshielded speaker wires are a great antenna.
The final issue is the inverse square law. A weak transmitter that is a few meters away from an aircraft system can easily jam a strong transmitter that is 20 km away.
Under the right conditions, almost any electronic/electrical device can be a source of interference. This can be greatly reduced by proper design, filtering, and shielding, but that costs money, and many manufacturers cut corners in these areas.