400mg is the upper limit of what a healthy adult should consume in a 4hr period. I can easily crush a Trenta Pink Drink from Starbucks in like, 25mins if I'm thirsty. But not all adults are healthy, in fact, most aren't. If you read my linked article, those drinks alone can cause caffeine toxicity in really short order. Especially for people using Panera to telework or study in a place with decent food, wifi, and free refills who just keep going back for more, unknowing of the danger they're putting themselves in.

It's insidious too, because while stimmed up on caffeine, they'll find they 'work better' when they're in a Panera, thus creating the wholly intentional feedback loop of wanting to work/study there more often.