For the same reason that this also issues the warning:

@array = qw[ foo bar qux ];;
print @{ array }[ 0 ];;
Scalar value @{ array }[ 0 ] better written as ${ array }[ 0 ] at 
foo
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But these do not:

$name = 'array';;
print @{ $name }[ 0 ];;
foo

$ref = \@array;;
print @{ $ref }[ 0 ];;
foo
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As this scrunched up deparse output shows, the extra level of indirection means that the code goes through a different path, and the warning has never been attached to that other path:

9  <;> nextstate(...)v ->a    9 <;> nextstate(...) v ->a         
g  <@> print vK ->h           g <@> print vK ->h
a   <0> pushmark s ->b        a  <0> pushmark s ->b
f   <@> aslice lK ->g         f  <@> aslice lK ->g
b    <0> pushmark s ->c       b   <0> pushmark s ->c
c    <$> const[IV 0] s ->d    c   <$> const[IV 0] s ->d
e    <1> rv2av[t5] sKR/1 ->f  e   <1> rv2av[t4] sKR/1 ->f
d     <#> gv[*array] s ->e    -    <@> scope sK ->e
-e syntax OK                  -     <0> ex-nextstate v ->d
                              d     <$> const[PV "array"] s ->e
                              -e syntax OK
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