More that people will form the wrong conclusion - vegetables are naturally prone to e. coli - because they didn't point out that thus had to come from an outside source.
No need to speculate if we've not nailed which source, but we can _highlight_ the current lack of actionable info rather than let people assume one.
CDC alerts have a fairly narrow, action-driving purpose; every extraneous word increases the number of people that tune it out, with negative public health consequences.
Complaining that they aren't adding information that—true as it is—is irrelevant to the task at hand to the alert is, quite literally, asking to make the immediate public health issue worse, and for more people to die.
No need to speculate if we've not nailed which source, but we can _highlight_ the current lack of actionable info rather than let people assume one.