Of course there is a dynamic between the business and the employee when it comes to their expectations of each other. All else being equal, a business would like to get more output per dollar spent, and an employee would like to get paid more in total and work fewer hours. Nowhere in the goals of this dynamic does hours worked come into the picture. What does happen is that businesses believe they would get more output per dollar spent if they can get a salaried employee to work more hours, so they pressure employees into doing so. People generally like to be in charge of others, so un-enlightened managers force employees to be at the office because they like seeing them there.
Enlightened managers care first about cultivating great relationships, secondly about the total output of an employee, and therefore not at all about hours worked. Marginal productivity per hours worked eventually goes negative as hours worked increase, and in my opinion the point at which it becomes negative is a lot lower than most people believe (probably ~20-30 hrs/week over the long term).
Besides, highly productive developers are in very high demand. You're just shooting yourself in the foot if you don't give them a fair deal, because they'll go somewhere else, unless they're on a work visa in which case they'll remember if you don't treat them well.
Please do note that this argument applies mostly to salaried employees in knowledge-work.
Ok, but "we expect you to work full time when we hire you for a full time job" is a fair deal. It is not "unfair" to hire people full time rather than part time.