They drive at mm/s speeds. Moving over a few meters because the soil is stirred up is a huge waste of energy and time, not to mention the risk of failure.
Actually, 30 m/hour is 8 mm/sec. But that's a bad choice of units, because seconds is not a good mission time scale.
The point is that, within a couple of hours, you can drive beyond the contamination radius. And, over the span of the mission, you most certainly will.
