That's what contracting is about. Getting hired quickly, usually getting higher hourly rate than employees with the downside of it being a temporary engagement and being the first to go when times are tough.
This would be like a newspaper being outraged about someone cancelling their subscription because they thought the subscription was safe.
That doesn't appear to be what Apple & Google etc are doing. The contractors aren't short-term troubleshooters or hired for a project. They're long-term quasi-employees, working alongside employees, often doing similar tasks as employees, just outsourced via a staffing-agency.
They're often paid less than employees (e.g. today's article [1] that includes a part on raters at Google alleges that they get $14 while Google's minimum hourly pay for any employee is $15), and the agency is the one taking the blame for mistreatment while the company can say "we didn't know! that's terrible!" and then get the next staffing agency, tell them exactly what they want, and have another fall guy to take the blame.
It's not about "we need a hundred people for the next 5 weeks on this, quick", it's about not hiring non-essential employees directly. Google will hire the PhDs, but not the quality raters or content moderators, because they are much easier to replace and there's no value to Google in employing them directly while they'd take PR hit if they were the actual employer (even though they're the ones saying what to do, when to do it and how to do it, which is often used as a test whether you're an employee or a contractor).
This would be like a newspaper being outraged about someone cancelling their subscription because they thought the subscription was safe.