It's not just free overtime.
That is often very much what being on a salary means in practice: you get X money per day/month/whatever, and X doesn't increase if you work more than your normal hours. However, it's also not unusual for salaried employees to have those normal working hours specified in their employment contracts, effectively putting a lower bound on the amount of working time expected in exchange for the salary.
Again, though, this all depends very much on where you are and how your local labour laws work. For example, the US system of at-will employment is actually closer to what most of the world would consider contract or freelance work than employment, typically involving very little commitment to continuing the relationship by either side and relatively low benefits for employees beyond their pay cheque. What is particularly unusual in that case is that in much of the US such an arrangement seems to be the norm even for entry-level and low-paid work. Elsewhere, employment tends to involve much more of a commitment from both sides for those kinds of jobs, while the lighter touch arrangements tend to be used more for skilled professional work. In that context, abusive hiring and firing is usually less of a problem, and all parties may benefit from the greater flexibility, including flexibility about compensation arrangements.