Why are standard cast iron maintenance practices compatible with food safety?
Toxic?

Neither the seasoning (which is essentially polymerized fat) nor the rust (which is... rust) is harmful in small quantities.  Of course, I would not eat either by the spoonful.

Spray oil as seasoning fat?

See the many questions on seasoning, but cooking spray is far from ideal as a seasoning fat, as it contains emulsifiers and such, and tends to be a poly-unsaturated fat (that is, liquid at room temperature).  Saturated fats are better.  One of the most common household options is, dare I say it, Crisco.   Or bacon fat.

Why is scrubbing with salt sanitary?

Tradition is that cast iron is not washed with soap and water.  The purists will rail at me for saying this, but an occasional light wash with mild dish soap is not going to irreparably harm your seasoning.  As you cook, it is continually being rebuilt, at least in frying which is what cast iron pans are best for.

Nonetheless, the common method of just cleaning out with salt and then drying leaves a dry surface, which is not hospitable to bacteria or other pathogens.  They cannot grow in a desert.  Even if there are micro-crevices in the pan (there are), they would either be filled with the seasoning layer and irrelevant, or leaving the bacteria in direct contact with that active iron surface, which is also not good for them.  But the main issue is that the pan is dry. 

Finally, since the pan is hot when being used (often even preheated in many applications) any tiny amount of bacteria which find a foothold will normally be killed quite early in the cooking process.

The seasoning&mdash;even from delicious bacon fat&mdash;that forms the protective layer on the pan is also polymerized, which makes it less available as a food source for bacteria, even if they could otherwise grow in the very dry environment of a properly cleaned and dried cast iron pan.