Every airline that I know will allow infants to travel in any class. While it may be a bit of a disturbance to find yourself sitting next to (or near) a screaming infant, at least in the front end you have noise cancelling headphones
What you seem to be saying is that its OK for those "down the back" to have to put up with screaming infants - but not those that turn left on entering the aircraft - why? Not just because of the money paid surely, there are many airlines whose full economy fares are almost the same as consolidated APEX J class.