Why? Let us read first the Hadiths used by Ms. Hascall below:
"Give a servant his fee before his sweat dries"
"God Most High said: I shall be the opponent of three people on the day of judgment: the man to whom I gave generously but then he cheated; the man the man who sold a freeman into slavery and ate up its price, and the man who hired a worker and took his due measure from him but did not pay him for his (fair) wages"
The word "fair" that I underline above is an additional language used by her. I have to disagree with Ms. Hascall because from the very beginning, these Hadiths do not deal with the fair amount of wages to be given to a worker. It is true that some Islamic scholars tried to argue in such a way, but I do no think that their interpretation is correct.
In my view, these Hadiths deal with the obligation of an employer to honor his contract with his worker, i.e. to pay his employee's salary/fee for the work that has been done and that he should not postpone such payment without a valid reason. No words on fair amount. Therefore, this is about the sanctity of contract not minimum wages.
As I have argued several times in my blog and in my paper here, Islamic Law (as a concept and not in the context of Legal Positivism) separates moral and legal issues and also puts efficiency and the general welfare maximization as the main principles in building its legal system. This is why Islamic Law does not prohibit or even condemn pre-existing slavery, condemns riba but does not provide any sanction even though God says that the sin of committing riba is equal to murdering a man or having an incestual relationship with our own mother. When dealing with economic/commercial issues, we have to admit that Islamic Law is very flexible, namely it does not criminalize the violation of provisions relating to commercial issues.
This is also in line with the fact that God does not prescribe an absolute value of minimum wage and cannot be expected to do so. Once we deal with fairness issue, there is no single clear answer. Saying that Islamic Law compliance can only be done once you pay your worker with a fair amount of salary creates too much ambiguity. It also transforms a moral issue into a complicated legal issue. As an example: if an employer pays his worker too small, does it mean that he violates the law? Would that mean that the contract is invalid? What would be the consequences?
Of course you can always say that it would be good if employers pay attention to the overall well being of his employees and should pay good salaries to them. But that should stay as a moral issue rather than a legal issue with all of its consequences. Because payment of salary is also subject to many factors and the law of supply and demand. As there is a fairness aspect relating to the employee, there is also a fairness aspect relating to the employer.
I think that is why Islamic Law focuses more on the enforcement of the contract to protect the rights of the worker and the Hadiths are more consistent with this approach. As I argued here, I believe that the best way of promoting the interest of the workers is by making policies that are correlating with the supply and demand of manpower.