Microsoft once (in 1998) required that its suppliers provide a few benefits to their 'temporary' employees

Especially important for people concerned about the present situation of 'permatemps' without any benefit provided by their staffing agency is the information given by Ron Lieber (read the extract below). It clearly shows that Microsoft at that time (1998) did require its suppliers to provide a set of benefits to their 'temporary' employees. Unfortunately I have not been able to find additional relevant information. My guess is that in the following years, for whatever reasons, the benefits requirements were abandoned.
August 2000: The permatemps contretemps, by Ron Lieber, Fast Company

"In other areas, Decker (Microsoft's HR person at the time) has had more leeway to improve the standing of temporary workers at the company. In 1998, for example, she instituted a platform of changes that requires every temp agency that works with Microsoft to pay for at least half of a temporary worker's medical and dental insurance, to give workers at least 13 paid days off each year, to grant them at least $500 worth of training annually, and to establish a retirement-savings plan with at least some matching contribution from the agency. And, for the first time, all temps in every job category have a choice between at least two agencies that they can sign up with. "If you're going to use a contingent workforce, then you need to make sure that there's a safety net in place for those workers, and you need to underwrite the costs associated with that net," Decker says, noting that Microsoft now pays 20% more per worker to agencies than it did just a year ago."

From Fast Company, August 2000 issue