People do think about U-turns.
Waze has some problems related to U-turns. The first is that there's no audio direction for U-turns, so if you are on a two-way segment, you won't get an audio command telling you take a U-turn. If you are on a divided road and the intersection is implemented as an H or #, then you'll get instruction to make a left, followed immediately by instruction to make another left. If the intersection is implemented as a bowtie, the instruction will be a single instruction to make a left turn.
The other problem is with the navigation engine. If U-turns are allowed, Waze tends to get very little data about people making the U-turn, so the segments have invalid speed data. This causes Waze to frequently inappropriately route down a street to make a U-turn and come back to the intersection thinking that this will be faster than going straight through the intersection. This is prevented by generally disabling U-turns except where they are actually necessary for navigation (and will therefore probably get valid speed data).
Orange County, CA, USA, Samsung Galaxy S3, Android 4.3