From what I can tell the LimeSDR ( and HackRF and bladeRF) sensibly chose that their maximum output power is under the legal limit. And hence are “Exempted devices” under FCC 15.103 subsection C in CFR 47 (Code of Federal Regulations).
“( c ) A digital device used exclusively as industrial, commercial, or medical test equipment.”
A tiny bit of knowledge is all that is required to increase the signal levels beyond the legal limits.
1 external gain (LNA/Broadband RF amplifier/MMIC)
2 .an external band pass filter to put a hard limit on all intended (and unintended) transmissions to keep them all inside the legally licensed band.
By making you provide external equipment, it puts ALL of the onus on the person doing so to responsibly find out and obey all local legal requirements. If you don’t know how to to do something with RF, then you probably should not do it, not until you get an Amateur Radio operators license, and as part of the training you will find out all, the local legal requirements within your country.
Because they are classified as test equipment with unintentional (and in this case intentional) emission of radiation under ~10dBm, they avoid costly certification (which they would not pass because they do not block with a locked down firmware TX on bands licensed for very large yearly commercial fees by governments - which would be different for each country), and avoiding those certification fees, reduces the price point for consumers. Limiting the output under the legal limit also means that you do not have the case where some idiot buys a device with TX capability and decide to break laws and do stupid things on licensed bands at high power either unintended ( no knowledge ) or intentionally ( probably flagged as terrorist ).