433 is part of the amateur radio band which requires a ham radio license to transmit legally there above a certain power level.

433 is also regulated under Part 15 rules which is unlicensed.  There will be some sort of ERP limit here that will include transmit power plus antenna gain.  There may possibly be an antenna height above ground limit too.  Assume low transmit power, below 20 dBm (maybe well below 20 dBm), and some relatively low gain antenna, too.  Without looking it up, I don’t know for sure what the exact rules are.  I’m a ham so I can run 1500 Watts there if I want.  I’m not sure if I have an ERP limit there or not, though.  I guess it more comes down to if I can satisfy the FCC RF field strength rules or not for how close the antenna can be to people, signage, fences, etc to keep from frying people and pets unnecessarily.  :-)

In short, 433 is legal here in the US for unlicensed use, but there are power limits.  The Part 15 devices should already abide by the rules.  If you modify things then it voids the Part 15 authority in which case you’re not legal to transmit with it.