The FCC has requirements for manufacturers to make sure that their radios output to-spec EMR. In addition to this, they've been working toward trying to stop people from being able to arbitrarily modify their radios.[1]

While (AFAIK) there isn't a regulation stopping someone from selling radios that have completely free software basebands, you can bet that the manufacturer will be prosecuted if users suddenly start outputting radio waves that don't follow regulations (suing users is harder than suing a manufacturer). As a result, there's a disincentive for manufacturers to ever sell free software radios (because by definition they would have to allow modification).

[1] https://www.infoq.com/news/2015/07/FCC-Blocks-Open-Source