The last sentence from 2.1.4: "Targets must be arranged so that shooting at them on an “as and when visible” basis will not cause competitors to breach safe angles of fire."
I read this as "if you can see it, it better be safe to shoot it". We actually spent quite a bit of time on the range portion of the CRO class dealing with this issue while debugging stages.
The reason I am aware of this is from a level 2 match last year. We had one newer shooter get DQ'ed for 10.5.2 while executing the dumbest stage plan ever. The CRO called me over and showed me what happened and it was upheld. Later on, I went to the spot where he engaged a target that was about 20 degrees behind him to the left and sure enough the scoring face of the target could be seen. I went back to the stage design section of the rules and found this gem. A knowledgeable competitor could of attempted to arb the DQ based on a illegal stage.
I also thought back to the many majors that I have been to and there has always been some sort of vision barrier, wall, no-shoot or something to keep a target from tempting an unsafe shot angle. And as for retreat stages, of course you can DQ by letting you your muzzle get ahead of you. That is on the shooter. But you don't have to build retreat stages that allow for unsafe angles of fire.