Discloser: I am not a rabbi but well just well-versed and eat kosher food and keep a kosher home. For an authoritative opinion a competent Rabbinic authority should be consulted.
Now back to the salt and a short lesson in preparation of Kosher meat (warning some gory or graphic detail):
Actually it has nothing with being kosher in or of itself. All salt is kosher unless it comes in contact with non-kosher materials or vessels. But for all intents and purposes, all salt is kosher.
Kosher Salt really should be called: "Koshering Salt." It is salt that is used to make meat kosher. Kosher beef and mammal meat is usually from the front quarter of the animal (there are a few exceptions depending on locale and custom). All of the animals need to be checked for damage externally and internally (no lesions, sign of mistreatment, etc.,)
Blood is considered impure in Judaism and animals need to be drained of blood during preparation. They are cut along the throat with a knife that must have no nicks, cuts or other deformities such that the animal dies fast, in minimal discomfort and drains of blood (for practical reasons) as quickly as possible-especially since the heart will still pump for a bit while. This gets out most of the blood. There is still blood trapped in muscles and tissue.
This is where Kosher Salt comes in.
The meat is soaked so that the water carries out and washes the blood. Then the meat is salted after soaking; the kosher salt in a crystalline structure pulls out almost all of the blood. Sure there is some left, but most of it cooks off.
Most kosher meat benefits from rubs and marinades-AND SMOKE--but with all that kosher salt left behind from the process, I have to wash all of my meat (except chicken) to get some more salt out or the rub just makes it far too salty.
Kosher itself has little to with cleanliness...directly...although no bugs can be in lettuce and must be checked-or even in the water supply.
More here: http://en.wikipedia.org/wiki/Kosher_foods
And about the bugs in the water supply: http://www.wwdmag.com/wwd/index.cfm/powergrid/rfah=%7Ccfap=/cfid/1457519/cftoken/17860028/fuseaction/shownewsitem/newsitemid/7199
I'm pretty sure Alton Brown covered this on the salt episode...but I don't remember...
The Kosher Smoker