At my company (a mid-size, well-known financial firm), H1Bs are definitely slave labor. They're paid CONSIDERABLY less than FTEs, and are generally disposable if they screw up. My former boss once had to fire 3 of them in a single week because they had obviously lied about their skills and didn't know what they were doing.
I know plenty of H1Bs. They live in constant fear of their job ending and having to be deported. Afraid to speak up, afraid to ask for a raise and hoping that the company will sponsor them for a green card.
A career and life choice they made. Opportunities here are likely better than their country of origin, so they (hopefully) weighed the risks and benefits of the choice to immigrate here for work. If those calculations no longer hold true, then likely a new course is to be chosen. Slavery implies force or coercion, not a regretted decision.
Shouldn't the anti-immigration folks be happy that Microsoft (an American company) is employing less foreigners? Isn't that their goal?