It's a rule. I just copied this from the My Way benefts page on hhonors.com:
"** Executive Lounge Access Policy
The following policy will apply at all ConradŽ Hotels & Resorts, Hilton Hotels & Resorts, and DoubleTree by Hilton™ hotels with Executive Floor Lounges. Executive Floor Lounges are available only at select properties.....If you are a Diamond Hilton Honors guest, you and up to one additional registered guest will always enjoy complimentary access to the Executive Floor Lounge, even if you do not receive a room upgrade to the Executive Floor. "