I've seen this play more and more in my 6max games and I just can't wrap my head around this play being 'optimal'.

I raise UTG 100bb deep as a fairly standard reg, folds around to the BUTTON who is a winning reg and he 3bets. Folds back to me and I 4bet to ~2.3 sizing, and villain calls with.... JJ.

I understand that he's flatting my 4bet as opposed to jamming to try and pick off my 4bet bluff hands postflop but is this play really profitable vs my fairly tight and standard UTG range as opposed to just flatting my initial raise with JJ?