The GT and GTL does not have this problem.
The K1600GA is speed limited to 100 mph.
IMHO the only reason BMW would do this would be because instability was detected in wind tunnel testing.
The GA design added floor boards which IMHO contribute to front wheel lift at speed. With the GA this is made worse because the Top Box is mounted so far back which again will cause the front wheel to rise at speed. This becomes very noticeable on any motorcycle in turbulent air around large vehicles, or in cross winds.