I don't think its that static. It's a much more fluid situation - one that is looked at in the macro.
If the salaries are greater, the profession is more attractive to talented people who might otherwise make more money in other professions. Sure, there are committed lifetime professional teachers that are going to teach either way. But with lower salaries, some that are in the profession may eventually seek to go elsewhere when they aren't making the kind of living they would like. Raising salaries helps retain existing teachers and drives interest from others who want to become teachers. Then, at that point, the school system is able to better select teachers based on merit rather than the fact that the teacher is willing to accept the low pay.
That's kind of the idea. You can't look at it as a snapshot.