Why are professionals not legally required to be compensated for overtime?

As an engineer, at least where I worked, you were always salaried and full-time meaning 40 hours per week. Quite often, you’d be expected to put in more time than this. 45 was not uncommon, for some 50, 60, 70 hours a week even were the norm.

The state law where I am specifically exempts professionals from the overtime pay requirement. Some companies do have a policy of paying OT, while others do not.

When you are first hired at a company, you cannot know for sure what the work load will be – you have to take the company’s word for it unless you know someone who’s worked there before. If you negotiate a salary based on a 40 hr week, then are later expected to work more, there is little you can do about it if the employer refuses to renegotiate your salary.

It is totally unfair to me. What is the history of this exemption and on what premise is it allowed?