MySQL: Why statement-based binlog format cannot work with Innodb READ UNCOMMITTED or READ COMMITTED Isolation levels?
The semantics of those isolation modes cannot be guaranteed in SBR since the Slave is unlikely to execute the statements of multiple threads in the same order.

More specifically, from the Change Log:

----- 2008-11-14 5.1.30 General Availability -- Bugs Fixed -- -----

With statement-based binary logging format and a transaction isolation level of READ COMMITTED or stricter, InnoDB printed an error because statement-based logging might lead to inconsistency between master and slave databases. However, this error was printed even when binary logging was not enabled (in which case, no such inconsistency can occur). Bug #40360