Max LTE bandwidth (~10mbps) times 30 days is a bit over 3 TB of data. Verizon is being honest about the fact that they'd never be able to provide that. Neither could the other carriers, their marketing just ignores that fact (or hides the fact that their plan isn't really "unlimited").
They're also saying it's highly doubtful that you NEED 3TB of cellular data per month. If you do, it's not unreasonable that you should have to pursue a networking solution different than what your mom uses to browse Facebook on her iPad.