Dig we must.
Like I said, I'm sure they've given this some thought. But even so, the hot, toxic blast of a rocket exhaust might disturb soil quite a ways down, perhaps causing surface ices to melt into it, changing pH, destroying potential life, blowing surface layers about, etc. Perhaps the jets are angled away from the spacecraft, or they are designed to cut off well above the ground and rely on Mars's low g to cushion the landing, etc.