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