Seems that postfix for actually creates a loop. I wonder if that has always been the case.

$ perl -MO=Concise,-exec -e'while (foo()) { bar() }'
3  <{> enterloop(next->8 last->d redo->4) v

$ perl -MO=Concise,-exec -e'bar() while foo()'

$ perl -MO=Concise,-exec -e'do { bar() } while foo()'

$ perl -MO=Concise,-exec -e'for (foo()) { bar() }'
8  <{> enteriter(next->d last->g redo->9) lK

$ perl -MO=Concise,-exec -e'bar() for foo()'
9  <{> enteriter(next->d last->g redo->a) lK
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(Output filtered using grep 'enterloop\|enteriter')

bar() while foo() uses a goto instead. It is implemented almost identically to:

LOOP_START:
   ( bar() ), goto LOOP_START if foo();
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do { bar() } while foo() is not readily representable in Perl, but it doesn't use a loop either. The closest I can get is

LOOP_START:
   do { bar() };
   goto LOOP_START unless foo();
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But yes, the answer is "Implementation Details". There's no real reason for it not to work except backwards compatibility.