433MHz is attractive because it's low frequency allows it to propogare farther with less energy input than higher freqs (900MHz; 2.4GHz) would need and does not suffer from nearly as much reflection off of obstacles thanks to that longer wave.
There are several frequency ranges in the US that are unlicensed for transmission. But don't confuse unlicensed with a lack of rules governing what you are allowed to transmit; how often you can transmit and for how long. Because you can plop a 433MHz transceiver into anything, you want to be careful that you're not clogging up the local airwaves by not knowing to know the rules. Also, most smart meters (near me anyway) operat in this band sending out pulses every so often. They mesh together to relay the data towards a central collector. Thanks to that low frequency, hundreds of meters can be visible at times showing up as tiny chirps all over this area of the spectrum. Unfortunately this also means that some cheap receivers (just looking for any signal on a very specific frequency in that range, can be randomly triggered by this 'noise'. But also, because it's used by utilities, to want to make sure they don't end up having an issue with meter readings because you began running a wifi link over 433MHz.
There are several frequency ranges in the US that are unlicensed for transmission. But don't confuse unlicensed with a lack of rules governing what you are allowed to transmit; how often you can transmit and for how long. Because you can plop a 433MHz transceiver into anything, you want to be careful that you're not clogging up the local airwaves by not knowing to know the rules. Also, most smart meters (near me anyway) operat in this band sending out pulses every so often. They mesh together to relay the data towards a central collector. Thanks to that low frequency, hundreds of meters can be visible at times showing up as tiny chirps all over this area of the spectrum. Unfortunately this also means that some cheap receivers (just looking for any signal on a very specific frequency in that range, can be randomly triggered by this 'noise'. But also, because it's used by utilities, to want to make sure they don't end up having an issue with meter readings because you began running a wifi link over 433MHz.