Watts per kilo is not a good way to determine your category for races. Someone who is over 4 watts per kilo is made up to an A even though they finish middle of the B pack. The winner is well under the 4w/pk and minutes ahead of riders who are over the limit. So could Zwift not come up with a different way to categorize riders so the racing would be better, A riders best lap time would be far superior. Each rider could have a category in there profile for flat, hilly, and mountain. They would then know the category to enter.
I feel that Zwift have a duty to try to sort this out rather than leaving it to race organizers to sort out. Races are part of the Zwift experience.