Calling baby diapers as diapers and adult diapers briefs doesn't really bother me in the least. I can certainly understand that incontinent adults would not want to associate diapers with being a baby, so I am assuming that would be one of the reasons for the different terminology.
Also, there are different types of adult absorbent products, whereas for babies, they are all diapers. Adults have a wide selection of tape on briefs, pull ups, belted undergarments, guards, pads, etc, so most of them could be referred to as "diapers" but each has their own description, at least that's how I see it.
Another thing I've noticed is that the adult diapers (labeled incontinence or adult hygiene) are never found anywhere near the baby diapers. There does seem to be some fear of acting like the two are in anyway similar.