Because after the foul (a) defenders hesitate expecting it to be given, (b) they may be out of position, (c) with a free kick they will have everyone in defence and (d) City expect to play on. It's why our "fouls against" numbers are way lower than the number of fouls - not even the one where the offender got booked a few minutes later would count in the stats.
I don't see how you can criticise Oliver for applying advantage when that's how we scored the first. Did you not notice his "celebration"?