I flew from Baltimore to Boston last month on JetBlue, and I too was designated a Pre-Check passenger. I asked the young woman at the ticket counter if she knew why, and she offered the opinion that I was selected because I am a -- ahem -- 'more mature' passenger.
Thinking about this, there are some things I think we can conclude...
First, you are first 'screened' when you book a flight. (See
https://www.tsa.gov/stakeholders/secure-flight-program ) People who are judged to be security risks are probably not going to be allowed to fly at all. So if you are issued a boarding pass, there is already a high level of confidence that you aren't a bad guy.
Secondly, if you're an older person, an American citizen, and you are in the airlines' databases as an established customer, there is very low likelihood that you are going to try something sinister at the airport.
Thirdly, letting folks like me and Sip breeze through screening speeds up the whole process, and lets TSA focus on people who might be a real threat to security.
Bottom line: The knock on TSA has always been that they don't use common sense. Well, it looks to me like this is exactly what they're doing with the Pre-Check program: Using common sense to everyone's benefit. Good for them!
