Without carrier permission, they might be violating some type of interference regulations, but I would also think the fake cell device itself and maybe its operator would need FCC licenses regardless of carrier permission. As an Ars Technica post I linked in another comment shows, the cell sites are probably being used outside of the constraints of their FCC licenses.
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Because without carrier permission, they're violating Federal laws administered by the FCC.
Without carrier permission, they might be violating some type of interference regulations, but I would also think the fake cell device itself and maybe its operator would need FCC licenses regardless of carrier permission. As an Ars Technica post I linked in another comment shows, the cell sites are probably being used outside of the constraints of their FCC licenses.