OAP
Not sure how long ago you were a pilot, and I strongly suspect from your comments you have very little understanding of the world of military aviation, but just because you don't understand a concept, please try to avoid telling the world it's wrong!
You clearly can't comprehend the reasons stated for using TCAS in TA-only. I can do no more than tell you that many RAF aircraft fitted with TCAS will at times be required to fly close enough to other squawking aircraft which would cause, when operating in TA/RA, an RA to occur; they'd be forced to follow an RA that would move them away from another aeroplane that needs to be close!
It's not appropriate to divulge many of the numerous tactical situations where an 'inadvertent' RA could occur, but for a peacetime one have a google of 'Queens Birthday Flypast'. It may even be on YouTube. Lots of aeroplanes all quite close to each other flying through the middle of the London TMA. Now tell me how that would work if all the heavies fitted with TCAS flew in TA/RA? I believe one year we even took the entire flypast through the Brize overhead. I don't recall ATC asking or even expecting us to be in TA/RA!