A quick peek within the source revealed that it does quote table names and other values that you can't use placeholders for.

SQL::Abstract does quote table names and field names with whatever $self->{quote_char} is set to, which is default to empty string. At least MySQL allows to quote table and field names with backtick characters (`table_name`).

use SQL::Abstract;
my $SQL = SQL::Abstract->new(quote_char => '`');
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Quoting tables and fields (selet * from `user`) is different from quoting values (where name = 'bob'). So what's the problem? You can't use placeholders on tables and fields, after all, only values. From DBI docs:

With most drivers, placeholders can’t be used for any element of a statement that would prevent the database server from validating the statement and creating a query execution plan for it. For example:

  "SELECT name, age FROM ?"         # wrong (will probably fail)
  "SELECT name, ?   FROM people"    # wrong (but may not ’fail’)
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