I am Jack's total lack of surprise.
Certainly there are so vanishingly few legitimate reasons a persons phone would be discharged...
Certainly there are far fewer reasons a person would want to go to the USA anymore. Or, rather, people value their dignity more than US culture; That you continue to have a tourism industry is beyond belief. Further, with Germany setting the standard for tearing US businesses out of their public infrastructure I'd be surprised if the US continues be a player in international business for much longer.
Anyway, to answer your question about why my phone would be discharged, it's because I'm forced to wait for three hours in the damn departure lounge because getting through security takes an age. I pass the time by browsing the internet, listening to music, watching streaming video... On my phone.
Variables don't; constants aren't.