I'll agree that flat bars put the riders wrist and arms at a non-neutral angle instead of a "natural position", but I think 90 degrees is an exaggeration. My hands don't naturally hang flat at my sides, and they're certainly not perfectly vertical when I raise my arms in front of me.

Mary bars/3 speed bars have a significant angle that many people (including myself) find very comfortable. Different Northroad bars have different degrees of flare. Randonneur bars, Salsa Woodchipper bars, WTB Dirtdrops, On One Midge, and others offer different angles. Probably the most comfortable angle will vary person to person and be different at different shoulder angles.