In defense of the traffic engineers, this is usually a safety issue. These "No U-Turn" signs are generally placed at locations where drivers would be forced to merge onto a high-speed roadway at a point where there are no acceleration lanes. Or at locations where a driver would need to cross multiple lanes of oncoming traffic.
All it takes is one accident involving a knucklehead with a good lawyer -- without those signs in place, the first line of questioning in the civil suit would be as follows: "How could you NOT have a 'No U-Turn' sign in a location like that?"
The boneheads who do make the dangerous manuevers don't even look at the signs! There are some on-ramps nearby (US 101 south of San Francisco) which can send you in the wrong direction if you take the sign literally. If you go with the flow of traffic in order to avoid a physical confrontation with another vehicle, the next offramp is set up to prevent you from reversing direction on the freeway...you have to exit right, turn left onto a side street, go three blocks and turn around in a parking lot in order to reverse course. I've seen the same thing all over the country, so I don't think it's unique to California.
A simple mistake compounded by unclear signage may send you miles in the wrong direction, unless you commit a minor infraction. Plus the local police are often aware of this and set up ambushes when they need to write more tickets...whereas if you ask them to enforce stop signs and speed limits in your neighborhood - good luck!