Suspicious EXPLAIN output for a derived table + WITH + joined table

DROP TABLE IF EXISTS t1,t2,t3;

CREATE TABLE t1 (a INT);

CREATE TABLE t2 (a INT);

CREATE TABLE t3 (a INT);

INSERT INTO t1 VALUES (1),(2),(3);

INSERT INTO t2 VALUES (1),(2),(3);

INSERT INTO t3 VALUES (1),(2),(3);

EXPLAIN SELECT * FROM (WITH a AS (SELECT * FROM t1) (t2 NATURAL JOIN t3));

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                           |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

|    1 | PRIMARY     | t2    | ALL  | NULL          | NULL | NULL    | NULL |    3 |                                                 |

|    1 | PRIMARY     | t3    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer (flat, BNL join) |

|    2 | SUBQUERY    | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 |                                                 |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+