Because the evidence demonstrates very clearly that she is. I have once believed Jeremy was probably was guilty and at the same time acknowledged Julie as a liar. I follow the evidence my stance cannot change because the evidence does not change.
If you believe Jeremy is guilty and you believe Julie is truthful then she is an accomplice to the crime, not a victim. One way or another she is a liar. Then you have the cheque fraud she committed without any known influence from Jeremy. The woman's testimony is reprehensible whatever way you look at it.
I agree that it appears she is an accomplice to the crime. She can also be seen to be a victim of a very calculated and cunning man.
This isn't the first case where a possible accomplice has turned "Queens evidence."
Current guidelines state;
"Only in the most exceptional cases will it be appropriate to offer full immunity." Maybe it was decided this case was exceptional?
http://www.cps.gov.uk/legal/p_to_r/queen_s_evidence_-_immunities_undertakings_and_agreements_under_the_serious_organised_crime_and_police_act_2005/Assessing reliability and credibility11. A prosecutor should endeavour to assess, in cooperation with the investigating agency, the reliability and credibility of the potential witness. In doing so, prosecutors should consider, amongst other things, the following:
a. the consistency and accuracy of the accounts provided (this should be assessed by analysing the debrief material against other available evidence, information and/or intelligence);
b. the readiness of the potential witness to change their account (consider also the frequency of change and any explanation provided);
c. any deliberate distortion of the roles of others;
d. whether the potential witness is minimising their own role;
e. the scale of Bad Character - have they led a 'life of crime' and is there material to demonstrate a propensity to lie?
f. the honesty, motivation and demeanour of the potential witness (this should be explored in debriefing and the views of the investigator will be important in making this assessment);
g. any tendency to confuse (although might this just be a consequence of the potential witness' long-term criminality);
h. any issues around the medical health of the potential witness, following any assessment made of their medical records.
http://www.cps.gov.uk/legal/p_to_r/queen_s_evidence_-_immunities_undertakings_and_agreements_under_the_serious_organised_crime_and_police_act_2005/#a04aIt does not appear Julie Mugford "coerced or incited" Jeremy Bamber to take part in the murders.