Loose coupling does not require a mediator to route messages, although that is dominant ESB architecture. This is also a requirement within the JBI specification. The ESB model should not restrict the SOA model, but should be seen as a concrete (possibly narrower) representation of SOA. As a result, if there is a conflict between the way SOA would approach something and the way in which is may be done in a traditional ESB, the SOA approach will win in JBossESB.
Therefore, in JBossESB, mediation will be a deployment choice and not a mandatory requirement. Obviously for compliance with certain specifications it may be configured by default, but if developers don't need that compliance point, they should be able to remove it (generally or on a per service basis).