Honestly I don't find this super upsetting.

Growing up I was always overweight and I HATED shopping for clothes because most retailers didn't carry clothing cut to fit larger bodies. Most manufacturers just had the "wider will work" idea for plus size clothes. We would spend all day going store to store until eventually I got frustrated and settled for stuff that didn't quite fit perfectly.

 It seems as if they are trying to distinguish between standard sizes that just get wider as they go up in size and clothing that is cut to fit a plus size body.  Yes the sizing isn't dissimilar but I would wager that the Girls Plus sizes fit VERY differently than the "standard".

 I don't see it any different from them identifying Big&Tall and Women's Plus.  It gives you an idea of where to start your shopping journey.  

 Given the modern retail climate the fact that they have size varieties is great, even if the signage is a bit dated.