A large portion of SDRs have no FCC equipment authorization at all, and so don't necessarily comply with regulations---including the lack of an AMPS lockout. This makes them a little bit iffy for sale in the US, but there is generally a rule that allows "test equipment" to be sold without equipment authorization under certain conditions on its use. I (not being an expert on this, I am not a lawyer, etc) would describe most hobby SDR use as being a gray area, but one that is probably not of too great concern since most hobby SDRs on the market are receive-only and obviously these weird part 15 rules about scanning receivers don't really matter in the modern age.

SDR transceivers like the HackRF are probably still not being purchased by people who will cause any trouble, but I do worry a little bit more about unintentional disruption of important radio applications like aviation navaids or whatever. If I were to take a policy angle here, I think it might be a good idea to restrict such devices to people with amateur radio licenses since they are not especially hard to obtain (DE AE5JL). I'm sure there's a thousand people here who would vehemently disagree with me on that though.