I just checked out SRAM's site and found this:
http://www.sram.com/_media/techdocs/..._S7_2_2002.pdf
Page 12 says,
"Caution:The Spectro S7 hubs are not suitable for tandems,tradesmen's delivery bicycles,and similar." .
I couldn't find anything on actual weight limits. I'm thinking the rep was just being overly cautious. I also nosed around Shimano's Nexus site and didn't see anything there about weight cautions.