Under T&C normally they can.
Now you also have to be truthful here, how did you get salt around the tires. If you've been using the car off-road (they can tell, it's their job), you should have cleaned it and cover up your mess (and sometimes they can still tell, it's their job). Cleaning fee will be the least to worry if you breach the rental agreement.
If this was just driving normally, I suggest calling SLC manager first, then their corporate. I've never heard of anyone charged for cleaning under normal use. Now smoking or spilling coffee, that I've heard few times.
Last resort is file a CC dispute.