That is a great question. I can understand see both sides of the equation, but I lean towards the "no" answer. Let me give my rationale.
1. This would give too much power to corporations and on the flip side, the people would have little rights. If corporations are allowed to give whatever they want, as long as people are willing to work for it, then people who are down and out may work for very little. This is not fair. Corporations, then, would only be exploiting a bad situation.
2. I would also say that the government is also sort of on board with this logic. There is a minimum wage to protect people.
In short, we need to protect people, in my opinion.
As long as the hiring and raises are truly given based on willingness to work, then yes, I believe employers should have this right. If, however, the employers use this as an excuse to treat one group or another preferentially, their actions should be illegal.
I assume that you are talking about women because you use the term "equal pay for equal work." This is a difficult issue because of the fact that women might need to take time off for pregnancy and child birth.
Even so, it seems wrong to tell a business that they can not promote people who are willing/able to work longer hours or more consistently.
In most of the country the employers are allowed to fix the wages of their employees with two restrictions.
- no employee must be paid wages which is less than the legal provision of minimum wages or any other similar applicable law.
- There should be no discrimination in payment of wages based on caste religion or sex. This provision is described as equal pay for equal work.
The second of the above two provision is somewhat difficult to enforce by authorities because the the principle of equal pay for equal work does not prohibit payment of unequal wages for similar work to account for difference in capability and performance, and in general the employees have the discretion to assess performance and capability of individual employees and fix their wages on that basis.