Don't know, but it wouldn't be the only one. For example, list is a no-op in list context

PP(pp_list)
{
    dVAR; dSP; dMARK;
    if (GIMME != G_ARRAY) {
        if (++MARK <= SP)
            *MARK = *SP;        /* unwanted list, return last item */
        else
            *MARK = &PL_sv_undef;
        SP = MARK;
    }
    RETURN;
}
[download]

so why is it optimized away for being used in list context?

>perl -MO=Concise -e"$x = (1,2)"
8  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
7     <2> sassign vKS/2 ->8
5        <@> list sKP ->6
3           <0> pushmark v ->4
-           <0> ex-const v ->4
4           <$> const[IV 2] s ->5
-        <1> ex-rv2sv sKRM*/1 ->7
6           <#> gvsv[*x] s ->7
-e syntax OK

>perl -MO=Concise -e"@x = (1,2)"
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
9     <2> aassign[t3] vKS ->a
-        <1> ex-list lKP ->6
3           <0> pushmark s ->4
4           <$> const[IV 1] s ->5
5           <$> const[IV 2] s ->6
-        <1> ex-list lK ->9
6           <0> pushmark s ->7
8           <1> rv2av[t2] lKRM*/1 ->9
7              <#> gv[*x] s ->8
-e syntax OK
[download]

I would expect some consistency.