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

Memory Lane: two excellent articles by Samatha M Shapiro about the 'permatemps' at Microsoft... 15 years ago

 The Temp's rights was published in February 1999 and Temporary Victory in May 1999, both in The Stranger. Samantha Shapiro is now a contributing writer with the New York Times and many other magazines. 14 years later, the Microsoft's temps are back to square 1.

The Vizcaino victory appears to have been quite temporary.

The Vizcaino vs Microsoft decision, 9th Circuit Court of Appeals, May 12 1999

If you want to revisit the Vizcaino decison, rendered May 12 1999 in San Francisco by the 9th Circuit Court of Appeals: click Here or read below, especially at the end. Here is the résumé of the case by the plaintiff's attorneys  as posted on their website. They refused to discuss the case with me.