I don’t know if that’s even actually true, but it is certainly not remotely obvious.
Are they saying it’s illegal in the same way it’s illegal (in some places) to claim an item of value off the sidewalk? Is there a special new law covering these boxes? Or are they just bullshitting?
That last would be totally believable. The mall near me has signs in the outdoor parts saying “it’s illegal to smoke here”, which AFAIK is a flat-out lie – even if the owners intend to eject you or sue you for doing it, it’s not a crime. Cops and fake cops will absolutely pull imaginary laws out of their asses if they can’t think of a real law to explain why you have to obey their whims. And I’d imagine it will take some cops in the US a long time to fully grasp the concept that cannabis is not illegal.
