It sounds simple until the time comes to figure out how to actually implement it within the current map database structure (which is based on individual segments), and start considering some exception-handling.
How long does that "35" last? Does it automatically get applied to every segment that the Wazer drives until he changes it to something else? What if he forgets and leaves it off in a school zone, and there aren't a lot of Wazers in the area to set the record straight? Some speed limits change according to the time of day, day of week, class of vehicle. I'm sure there are many more.
This requires time & effort to design, implement, test, deploy, and maintain on an ongoing basis--time & effort that would otherwise have gone toward finding better routes; handling tunnels or U-Turns; automatically identifying & solving problems with the map; rolling out map changes more often; making WME more user-friendly; leaving editor notes on the map; allowing editors to communicate with Wazers who report problems...
There are so many more important things to work on, than gathering a continent's worth of speed limit data that Waze doesn't need, just to be able to put it on the client display, when it is already posted on the street.