I have the fun of going through Secondary every time I touch US soil... for me, if I have a connecting flight on the way from LHR to SFO (via Dallas, LA or Chic

I have the fun of going through Secondary every time I touch US soil... for me, if I have a connecting flight on the way from LHR to SFO (via Dallas, LA or Chicago) then I need to allow 3 hours for the connection. It is very hard to book such flights, and if booked as 2 single hops I've once before missed the connection due to being in security and United argued that the second flight was not a connection and so they didn't have to put me on the next flight. My baggage of course had gone ahead.