but not just for this publication. They tend to keep it short and sweet (although alternately, sites that I use like "Law & Crime" can go hog wild, where the full headline won't fit in the OP Title field).
And based on the timing of when this "warning" apparently happened, which was BEFORE the article was written, i.e., "in the past" (and not "real time" or "live" ), then putting the headline in the past tense as something that had already happened, is obviously more accurate.
"Issued" is a verb and "was" is also a verb so together they comprise what is dubbed a "verb phrase" (where the "was" is considered a "helper" ). So for headlines, both are not necessarily needed because "issued" is already past tense, and for the grammarians, probably something they could go on and on about. 
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