This might be a configuration option on <transaction registerSynchronization="true" .../>. This would be false by default.
Syncs have the advantage that TMs can optimise 2PC with a 1PC where only 1 XAResource is present.
Useful when used as a simple cache in front of a DB. E.g., as a Hibernate second level cache: if the cache would register itself with the TM as a XAResource than at commit time, the TM would see two XAResources (cache and db) and would not be able to make this optimization. This means that it would write to disk tx logs - time consuming.