Many of those devices are operating under the FCC's Part 15 rules for unlicensed transmitters, which allows them to use certain bands provided that they stay be

Many of those devices are operating under the FCC's Part 15 rules for unlicensed transmitters, which allows them to use certain bands provided that they stay below a minimum power level (or use an inefficient antenna) and don't cause interference to licensed users. The idea is that by keeping the radiated field sufficiently weak, you avoid long-distance propagation and won't interfere with anything not in your immediate vicinity. This is why your garage door opener only works when you're right next to your house. (Compare with a licensed mobile radio, which will easily transmit 100-300 miles given a clear line of sight.)