Can't speak for AL, but it was the norm in GA until this year. Now they are saying to keep ball boys off the field.
In reality, they are still coming on the field, but are usually stopping about the numbers. Most umpires WANT them to run the ball out to the middle, as umpires want to run as little as possible! But the new mechanic says keep them off the field.
I see a big difference between a 10 year old kid as the ball boy, and an injured player who is 17. The 10 year old may not understand when and where he is supposed to be (or more importantly, not be), while the 17 year old injured player probably gets it.
Problem is, when they don't come on the field and simply hand the ball to the wings, the wings then have to throw the ball into the U. Apparently, most wings were NOT former QBs, and I have seen some tosses go awry, and now we're back to umpires having to run after loose footballs, never a pretty sight.