I think litter laws should apply in this case. I'm afraid it would never get enforced because the US justice system tends to let the rich get away with anything, but dumping 200 pounds or more of commercial waste in Texas carries a minimum 180 day sentence under Texas state law (https://www.ennistx.gov/Illegal-Dumping-Violations/).
Not something I think should apply in the case of a true accident, or if you had a good plan to really go above and beyond in terms of mitigating the damage (not necessarily perfect, but very good).
Well, to be even more fair Elon doesn't seem interested in a diverter or trench but in a [thick] steel plate cooled by water. That would still allow the exhaust to run along the ground far beyond the OLM. He could erect some berms covered in concrete at a some distance from the OLM to avoid damage from that.
That said, I don't think it's that big a deal, and anyways SpaceX is already paying a steep price for this mistake.