You know, I've always heard that, too... that the reason they added the oval was for marketing and copyright purposes.

The part about it that I like is that they were able to explain it in "story" terms... I believe it was in Dark Knight Returns (I could be mistaken) where they actually mention that Bruce added the yellow oval around the bat as "a target" for anyone who would be shooting at him, provide them an area (other than the head or limbs) that was well-protected with body armor.

Rather than just change the costume for merchandising and copyright... there was a logical reason in the storyline.