Starbucks only sells cold drinks in trenta sizes, where the ice and/or milk is going to be a third of the volume. You can't get a 31 oz hot coffee. You are correct, you could buy two 16 oz coffees and get as much caffeine as Panera puts into their large lemonade, but if you ordered two 16 oz coffees, you know you're drinking two 16 oz coffees. The whole point of this lawsuit is that someone with a heart condition never expected lemonade to be jazzed to the tits with caffeine, and they died because of it.
Signs are only effective if people read them, and there's no reason anyone would bother to look for a warning sign on their lemonade. It's lemonade. Lemons, sugar, water, the simplest and most innocuous of all beverages. It's exactly what someone with a heart condition should feel comfortable ordering at any restaurant.
You can't trust orange soda or root beer or even seltzer because some brands are caffeinated, but lemonade is always safe. Except at Panera, where it has the caffeine of two energy drinks, enough to kill someone with a heart condition, because...