Where does the fault lie? - in an environment where they (the newsprinter) collect up front, have no responsibility for quality and arrogantly believe, perhaps correctly, that you'll be back with another paying job in the future.
Check the file you sent via Acrobat Preflight or Output Preview for RGB elements.
Check the file you sent via Acrobat Preflight - Checks - For Transparency Used
Do any of these elements off color involve rgb or interact (with or near) any transparency?
(InDesign could show these attributes via the links panel or a Live Preflight. Both can be (probably should be in your case) changed via the PDF Export)
