why not dump grey water wherever you want?
There are good reasons not to dump greywater wherever you want. Whoever that Forest Service person was that said it was o.k., should be re-educated in why the heck we have to reserve natural areas in the first place.
Not all plants need us to water them, even in a drought. Many plants can be overwatered and the roots start to rot, if not watered correctly, and at the right time. Acid-loving plants and plants that prefer to have a dry season can have tough time with greywater. We generally aren't botanists, so a good rule of thumb is to not dump water near native vegetation, or onto soil that doesn't percolate well.
Greywater is hazardous to our health, just as it is to plants and natural waterways. You wouldn't want to drink greywater. Most people use soaps and detergents that make us sick and are harmful to plants and animals as well. There are also nasty microbials that live in the pipes. Some jurisdictions have laws against kitchen sinks being included in greywater, due the the microbials like fecal coliform.
Greywater shouldn't be dumped arbitrarily.