Why would you think this?
That information is publicly available; e.g. http://www.myvisajobs.com/Visa-Sponsor/Microsoft/356252_Sala...
I'm not sure what the "average salaries" for non-immigrants are, but you can get an idea here:
http://www.salarylist.com/company/Microsoft-Salary.htm
It doesn't look that different.
Because that's been the pattern for most businesses. I doubt Microsoft is any different - why do you think it would be?
> It doesn't look that different.
The two pages you present compare "all MS salaries, globally" to "all MS H1B salaries, globally" - that's a meaningless comparison. A software engineer H1B is going to make more than a non-H1B tester.
Note again, I'm not trying to indict MS for anything here: I'm simply betting that MS is enjoying lower worker costs by employing H1B workers, just like many IT companies do.
I hope that's not the case, both for the H1B worker AND non-H1B workers.
On aggregate, H1B workers are being paid less than non-H1B workers. It doesn't take long to see this in action: http://www.foreignlaborcert.doleta.gov/performancedata.cfm#s... -- click around and ask yourself if those average wages look right. My state's average wage was what you'd pay a person coming straight out of college.
I'd definitely wager that if you compared all Microsoft H1B software engineers to all non-H1B MS SE's, you'd see a drop in wages. But I don't have a reliable indication of non-H1B salaries at MS, so I can't say for sure.