It works by toggling a GPIO pin on and off. This results in a square wave carrier signal, which means you get an infinite stack of odd harmonics - 3f, 5f etc. This means you're not just transmitting on the frequency you meant to, but on every odd multiple of that frequency (with correspondingly decreasing amplitude of 1/3, 1/5 etc) all the way up the entire spectrum.
It's very easy to accidentally clobber something like that. The only reason regulators haven't come down like a ton of bricks is the amplitude on PiFM is really too small to be very harmful - you're unlikely to mess up anything beyond the range of your own home, maybe your neighbors. If someone with a little knowledge tried to hook it up to an amplifier however, they're gonna have a bad time real darn quick...
> In most countries, transmitting radio waves without a state-issued licence specific to the transmission modalities (frequency, power, bandwidth, etc.) is illegal.
In practice, it's more like "if you get caught if it's illegal" --- and depending on what you're doing, getting caught can be very easy (e.g. broadcasting in an active band at enough power that your neighbours notice), or pretty much impossible (e.g. if you transmit inside a Faraday cage or at powers so low it doesn't reach the other side of the room).
Or as the old saying goes, "If no one else can hear you, they won't know you're saying anything."
The Nokia N900 had a FM transmitter, allowing you to stream to e.g. your car radio. Nowadays we got different solutions and this is mostly redundant (though still possibly of use in what we now call "older" cars). The transmitter was very short range, and therefore it was legal.
