It's kind of a dilution issue, and point source vs non-point source, and environmental responsibility... even some economics too.
A single tablespoon of fuel oil can contaminate upward of 2000 gallons of drinking water.
There are other sources of non-poluting weed killer, that have negligible environmental impact. using fuel as weed killer is also a tad wastefull... besides wasint the fuel.. you are wasting the energy expended to refine it.
Lastly accidental drips / spills, and other sources that are hard to prevent and track are non-point source contaminants. those are bad enough.. but we have to live with them. Point source problems.. like walking outside and dumping fuel on the ground can and should be prevented.. for the most part.. refined fuel is a non-renewable source of energy.
And for those that use waste oil for these types of chores... the epa and classified used oil as a hazardous waste product.
For leagal implications.. think about this. A contractor working in south florida was caugt using diesel fuel to control weed growth.. the two employees found got multi-year prison sentences, and the contractor was fined some astronomical amount of cash, and prevented from doing business with the state.. I think he also had to pay for water testing in the area for a couple years, as wel as have contaminated dirt removed, cleaned, and new dirt replaced.... I wonder what that did for his business potential?
You don't want grass in your drive? salt it.. much cheaper.
not soapboxing.. or lecturing.. just pointing out some info.
Soundguy