They "crashed" it at a walking pace, at worst it's a bit twisted, definitely still an archeological curiosity. Why do that? Probably to try to get close-approach data that the detachable probe failed to. Why crash? I don't know if the vehicle even has the capability to retro-thrust itself to a stop but trying would be a fiddly maneuver and would likely mess up the science (puffing up an opaque cloud of dust, blasting the landing site). So approach-and-keep-going makes sense. But then the vehicle will lithobrake with predictable smashy consequences. So it's toast one way or the other; turning it off is probably just tidiness.

All the above is guesses but seems to make sense to me.