If you have a heart condition or generally just a high sensitivity to caffeine, it's pretty important to know how much is a lot. The vast majority of people do not have any serious issues consuming 300mg or so of caffeine, so putting a bunch of big scary labels on this stuff just seems like overkill. For example, if you have a peanut allergy, it's generally on you to check the allergen list in the fine print, or if you have Celiac disease, you need to either buy stuff specifically labeled gluten-free or confirm with a restaurant, manufacturer, etc.

The fact that all of their signs have not just one, but three separate indicators ("charged", as much caffeine as coffee, and the specific caffeine amount) for anyone who might have a sensitivity is enough to show Panera's due diligence in my opinion.

I do get your point that people wouldn't automatically assume there's caffeine in Panera's random juice drinks, but caffeine is absolutely everywhere right now. I'd personally love for caffeine to be required to be listed in the nutrition label of drinks (or food that contains it) so you would know how much is in a Mountain Dew or Coke. But until that happens, I don't see how Panera could be seen as liable in this situation.