There's no discrepancy. RDS, being a managed service, does not enable every possible configurable value for every configuration parameter.
The architects of RDS chose not to make
STATEMENT available as an option for
BINLOG_FORMAT, presumably, because they recognize the fact that it has many limitations in a replication environment.
Not all statements which modify data (such as
REPLACE statements) can be replicated [correctly] using statement-based replication. Any nondeterministic behavior is difficult to replicate when using statement-based replication.
Even if you don't have RDS replicas, the binary log is still used for RDS point-in-time recovery. Without requiring at least
MIXED logging, there would be no way for RDS to restore an instance to a point-in-time and have it guaranteed to actually be identical to the original instance at the same point in time. Using
ROW makes that possible.
Indeed, the more confusing question is why the default value in the official distributions wasn't changed to
MIXED back in MySQL 5.1 or 5.5. Using
STATEMENT mode is almost always the wrong choice.