I haven't read anything about it but my guess is they realized people can't wear basketball jerseys in public and not look like a little kid.
By introducing these t-shirt unis they can sell "official NBA jerseys" at "official NBA prices" that an adult can wear without fear of exposed biceps, public ridicule, etc.
The theory about ad space is interesting (we know this is something they've floated before) because the jerseys already seem designed to look like Euro soccer "kits." I suspect from the design that they feel this could also be a better seller in Europe and other countries where soccer and soccer fashion is king.
Now you can wear an NBA jersey in Europe and not be confused for a hip hop enthusiast!