If you want to hold major companies accountable for the outsourcing companies they hire, fine, make a regulatory push for that, but you have to fix the outsourcing companies first before you can seriously argue for that or you're just making major companies pay for the problems of outsourcing companies.
Would you seriously argue for taxing people for buying from animal-cruel farms without actually doing anything to those animal-cruel farms?
That's the problem. Outsourcing companies are not FIRST in line to be fixed according to this article.
Those workers should be full time workers for the cleaning firm, but it doesn't mean that they are full time workers for Apple.
However, more often, the contractors are onsite and doing exactly what other Apple employees do and are managed by fulltime employees of Apple. In this case the law says those contractors should have been classified as employees of Apple not the contracting firm, even if their employment contract says otherwise. see Vizcaino v. Microsoft Corp
I really want to know what BigCorp will chose the first between a company paying janitors above minimum wage and a company offering them to cut their cost on janitors by 30% (but janitors gets only enough to scrape by). My experience is that they will jump on the cheaper offer (so someone can get a bonus), and the janitors can go fuck themselves - and so can the quality of their work. It happens all the time, and not only with janitors.
Arguing that the fault rests (only) with the outsourcing companies is the same thing as arguing the fault of people getting low salaries rests (only) on those people. It may be technically true, but it seems pretty naive to me.
The customer could choose to consider the treatment of the workers a priority in selecting service providers, and the service provider could choose to refuse to exploit their workers just to secure deals, but neither of them have any real incentive to do so. It's not illegal to shop primarily by price. It's not illegal to hire PT workers even when FT would be viable.
I do agree, though, that the problem is a lack of regulation; unfettered capitalism really does have a evil end game.