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I've never done such a thing myself, but I'd assume it's because gears 7 (.839:1) and 8 (.667:1) are functionally overdrive gears for cruising. You're getting more power to push faster in 6th (1:1) but running out of rpms, while in 7th you've got plenty of rpms left but the ratio is reducing your mechanical advantage for acceleration so you wind up losing speed against the ridiculously large drag you'd be experiencing (drag goes up exponentially with velocity). The ECU sees you're decelerating despite the gas being fully depressed, so it tries to switch you back to 6th for better acceleration to fight drag.
Basically, it appears BMW did a pretty good job of matching the car's electronic limiter to it's drivetrain and aerodynamics. Even with the speed limiter removed, you can't go faster in practice without significantly more power and torque or aero changes to offset the exponential increases in drag, so it doesn't matter much.
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