Browsing the internet, I read somewhere that Americans visiting other countries seem to have an obsession with being over prepared, an example being... carrying water and backpacks full of stuff. Almost as of they are preparing for extreme sports or Mad Max end of the world type of deal, but that's not so ridiculous as it seems. As I type the words you read, I have a backpack full of nothing but waters and a few aspirins and snack bars. Why, am I going hiking for the whole day? No, I'm going to the state Fair.
Here in the US, we give great importance to our hydration, I mean check out our cups! And most especially water in public places. It's not that don't trust the source or cleanliness, but the availability is something to be embarrassed about here in Public locations across America. At least California. Nevermind that fact that we're in a drought as it is, but we have been conditioned to be prepared for hours of thirst and walking, despite being in a crowded area. Where water fountains have waiting lines, taste funny and are far and few. That is why we carry stuff around like we're waiting for the apocalypse. Water!
I also think that it's kind of like tradition, the being prepared for anything scenario, where everyone has their own needs to think of and will not rely on the government aide of public services to help us, due to the fact that they let us down all the time. Because we're rebels.
Of course this is just my point of view.