So what are the salamandar ensatina? Aren't they still salamandars? And apparently interbreeding produces offspring with characteristics not beneficial so it isn't in their best interests to breed? If sub-species don't interbreed, isn't it usually because it would not benefit them?
If they interbreed, they're the same species. Also note that we tend to define organisms that produce infertile offspring when mated as different species (such as horse and donkey producing the mule.)
And apparently interbreeding produces offspring with characteristics not beneficial so it isn't in their best interests to breed?
That's not necessarily true. Hybrid vigour can occur, but usually specialized forms are more favoured than intermediate forms.
If sub-species don't interbreed, isn't it usually because it would not benefit them?
No. Organisms don't make these kinds of choices. They mate, and some offspring will survive better than others. If two different specialized forms can be favoured over time (i.e. darwin's finches with beaks specialized for different feeding types), this can eventually lead to speciation.
I'll also clarify: Many organisms DO make 'informed' mating choices (such as mate strength, showyness like the peacock, etc) but those mate selection behaviors will not change in an individual in the light of new information (say a new predator), instead, since certain new traits will be favored over time, then mate selection behavior that seeks out these new traits will also be favored.