Really? In my experience it is the other way round, at least for my taste. As the Brompton is back loaded from factory a mildy loaded front-bag evens this out. Plus if you carry a Brompton with weight on the rear carrier it is a bet wether the locking mechanism for the rear frame will give up or not while carrying.
It developed in an evolutionary manner, with the loads first always in the front. The rear was first used for spill-overs, but then it took off on its own, though admittedly with some overall reduction in the loads carried. The locking mechanism would likely first release while riding, as accelerations add to the gravity there, but it is a valid concern.
I guess it's just a matter of taste, and shoe size.
I didn't realise how often I flip the back wheel under the frame until I put something on the rack, and that option was ruled out.
I hardly ever flip the rear to park. I have a stable kickstand and it is far easier to activate than the flipping of the rear. When family got their kickstands, they also started using these to near exclusivity, so it does not seem to be just my personal contrariety.