Presumably because a range is a range, whether or not it's between A and Z, or * and {. From perlop:

    A character range may be specified with a hyphen,
    so tr/A-J/0-9/ does the same replacement as
    tr/ACEGIBDFHJ/0246813579/
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There's no mention of character ranges working only on alphanumerics. Although it does suggest that you shouldn't rely on it working properly for anything other than simple alphanumerics.