My bosses keep telling me that being on salary means I get paid to get the job done no matter how long it takes. That's how they justify not paying me time and

My bosses keep telling me that being on salary means I get paid to get the job done no matter how long it takes. That's how they justify not paying me time and a half for overtime. But if the job doesn't take 40 hours a week, then why should I waste time on busy work to hit that arbitrary number? If I can do it in 24 hours a week, or 16, or even 4 hours a week, then I should be able to just remain "on call" in case something comes up.