Sure. I can ask for salary history once the individual is no longer an applicant and if I can’t confirm they told the truth, I can fire them on the spot without cause. California is an at will employment state.
Seems like you've stumbled upon an opportunity both exploit your employees and bias your hiring towards candidates who don't realize you are exploiting them!
This seems like a really great way to ensure you never retain any decent employees ever. Who would stick around under an employer who would treat an employee like this, even if they had been totally honest about salary history?
See how long you last playing those games. Who in their right mind, other than the desperate among us, would work for a company that would do such a thing?
So yes you can do that now, but is this any different than saying you are going to be a bully just because you can?
Well, I punch people and take their money. Most people complain about morals, but hey this is how I earn the most money. You can't blame me for trying to maximize my profits.
I think this brings up a greater issue, and the purpose of the law in general - companies are motivated to be as profitable as possible, hence laws to prevent, say, a company building a militia and enslaving a state to turn them all into farmers or something (possibly the most profitable route? Who knows).