Hi,
I also observed this when using grails 2.5.0 which uses hibernate 4.3.8 .
I honestly think this should not happen since DB is in isolation mode TRANSACTION_READ_COMMITTED.
In hibernate documentation it is stated:

LockMode.READ acquired automatically when Hibernate reads data under Repeatable Read or Serializable isolation level. It can be re-acquired by explicit user request.

From this I would deduct that it should not be acquired automatically for READ_COMMITTED.
I would really appreciate if someone from Hibernate team could explain this behavior.
Thanks in advance.