This has been brought up before due to the pollution of the collected data by travel methods other than car.
At this point, waze has stated they are aiming at the commuter market and not the general gps market. So that is the reason this isn't in their from the beginning.
Also based on meetings with Waze HQ, they would prefer to programmically determine these issues and not rely on a settings which can be invoked in the wrong cases, or not turned off when they should.
Another reason which is my opinion, is that waze has the need to make this into a profitable product, which some of these avenues is selling the traffic data collected. A mode where use of the services is provided with no return goes against the ability to sell this data and pay for the service.