The error I get is the same as described in this issues: https://issues.jboss.org/browse/JBPM-3282
org.drools.persistence.jta.JtaTransactionManager is always used by default, with or without specify JTA. In fact, the exception occurs in that class, in the method getStatus.
In the last two messages in this thread, Marco Rietveld justified because jBPM need JTA: http://community.jboss.org/thread/168840
The author of the issue and thread, Marco Piraccini, has found a drools's issue for asked compatibility without JTA, but it is marked as "wont'fix."
https://issues.jboss.org/browse/JBRULES-2289
Everything indicates that we need JTA if we want to use jBPM.
Greetings