That's an interesting thought, but one I would take issue with.
It takes very little snow for the radar unit to become obstructed. Take a look at the picture I posted this evening in the sister thread to this one--the thread that was to be for pictures only. That was the result of about 20 minutes of driving through very light snow--really no more than flurries. At the point that I reached the highway the highway was completely clear, it wasn't snowing, and traffic was moving at 65-75 MPH. This was in no way an aberrational example. It takes extraordinarily little to obstruct the radar unit.
Also even when conditions dictate a slower rate of travel, we may want to set a constant speed. There's nothing inherently dangerous in setting a cruising speed on a wet or even on a snowy road. In fact I could argue that it might be safer than using the accelerator pedal, as I can make certain not to exceed whatever speed I determine is appropriate if I am able to set that cruising speed, whereas if I am using the pedal I may find that I am inadvertently travelling faster than I had intended.
There really is no good reason for Tesla to not provide functionality that exists in just about every other car on the road, and certainly exists in any other luxury sedan.