If you type in 'prevent u-turns' on the support page, this is returned :
Waze doesn't support currently u-turns.
In order to un-allow u-turns (in cases where the navigation might allow a sort of a u-turn by making to left turns) - the solution is to make the connecting segment shoter than 10 meters.
Unfortunately there isn't a way to mark a u-turn as possible.
It's the same answer at http://www.waze.com
and at world.waze.com, so they definitely link back to the same info behind the scenes on this topic.
Are any of you USA folk seeing an increase in recommended u-turns after any of the recent updates to either the client or new WME. Your new infrastructure's been in place for a while now, so not sure if anything has changed on ROTW only in terms of routing calcs.
Maybe it's just getting more visible with having more users in the areas ?