I am also of the opinion that some of the most ethical folks I have met are engaged in both measurement and mitigation of radon. I encourage the use of do it yourself kits, and if there is question of the results or if the results are adverse, that you call the experts. Get two or more bids and compare them. In the past, such was the case. Currently voluntary certification is available from two voluntary agencies.
Most states have mandatory certification programs.
We suspect that some of the public are overly impressed by lots of credentials in some folk's efforts to dress up like they know something that others do not. Kinda like the biggest lawyer ad in the yellow pages. Folks who graduated from Harvard will never tell you that they did. It may not be easy. I suspect that the record-keeping of the state is frustrated by some measurement and mitigation guys' desire to keep their success quiet and private.