With vegetables they are part of a regulated legal industry that has the ability to perform nation-wide awareness initiatives & recalls if there is some form of contamination.

And events like that also represents a significant financial loss to the producers, which itself acts as a form of accountability above & beyond regulatory penalties that might apply.

Basically, it doesn't matter if microbial contamination of weed isn't any more dangerous than for vegetables because current agricultural products have a remediation process in place to mitigate the danger, a process that doesn't exist for unregulated weed.