re: "it would just not be doable to do all that for every person coming to the U.S."
I can assure you that border and immigration officials in Australia, Canada and the United States do NOT check the cell phones, text messages, email, paperwork found in luggage, etc. for "every person" entering those countries.
These practices are applied very selectively. The immigration officials who work at customs desks and points of entry are trained to ask people questions and look at their paperwork and look for signs of deception or evasion. Only when they suspect somebody is being deceptive or evasive will they pull them out of the regular lines and question them further. When they think that the information the incoming visitor is providing (in their paperwork or verbal answers) is suspicious or inaccurate, then that is when they do a more thorough investigation.
So the more thorough investigation techniques are definitely NOT applied to "every person" entering these countries.
It is not difficult to find the story, which is a couple of years old: http://www.cleveland.com/travel/index.ssf/2015/08/babysitter_versus_the_border_p.html
Regarding border control for animal and plant products - Australia and New Zealand are very strict because of their island nature. NZ is even stricter than Australia. And people don't think about the massive impact that introduced pests can have. When I travel home to NZ to visit my family and then return to Australia, I have to declare that I have been in a rural area because my family home borders a farm. And I'm very careful about food I transport between the two countries.
Knowledge about the system and behavior of their border control, personal experience--going to the U.S. eight times on different visa types and entering on different entry points...Border officers just don't talk like that and don't have the luxury to go through tweets of some random German teenager just for this-- suspicion that she is about to commit a grave sin of babysitting for her aunt...