Actually, no need to have a boggled mind as this is very typical of large HS federations. The rule was set to help encourage financially disadvantaged upcoming lifters and school districts.
The reason being that, believe it or not, at a lot of these high schools, one can actually letter in PL, and they have large teams.
It becomes financially prohibitive for the school to provide this level of gear, and certainly not all students can afford new bench shirts.
Therefore not allowing bench shirts kind of levels the playing field, in particular as it relates to new lifters.
If a lifter wants to compete in a shirt, all they have to do is enter an open meet.
You may now remove your tin foil helmet.