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 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
“Anyone who believes in indefinite growth in anything physical, on a physically finite planet, is either mad or an economist.” -- Kenneth E. Boulding