Fork mounted bikes are being loaded more similar to their intention than being clamped in the middle of the frame tubes. Think about all the forces when you're riding, they are concentrated at the hubs through the axles, right where the roof rack skewers clamp. But the middles of bike tubes are never supposed to see anything like a squeezing.
Think about the tens of billions of miles driven with bikes in roof racks over the years, I'm not sure I've ever heard anyone say their frame or fork dropouts failed on the highway. I'm betting there is infinitely more damage from running into garage doors than anything road wind is doing.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein