Re passwords.
If you have nothing to hide, you have nothing to fear.
But what is it we have to hide, and, importantly, from whom? What if you have a dying relative in the States but are refused entry because you've posted criticism of the "gag order" on Mumsnet? What if you want to get a job in the States but can't make the interview because a friend of a friend posted "Trump is a moron" on FB?
This isn't about profiling, data gathering or snooping. It's about giving every jumped-up, racist, misogynistic immigration control officer the opportunity to turn you away just because he or she doesn't like the look of you. If Trump sets the bar for exclusion low enough, and removes the need for official channels to be followed, many thousands of "normal" non-US folk will find their important travel plans held hostage by little Napoleons all over the US.
For non-US nationals desperate to start a new life and who are almost certainly not dangerous, but who have made unguarded comments on social media that would be protected by the Constitution if they were American citizens, it opens the door to extortion. With a blanket ban or a points base immigration system, you either get in or you don't. With social media screening, immigration control have the option to turn a blind eye if it's made worth their while to do so.
How much is it worth to a potential immigrant fleeing civil war for that one unfortunate tweet to be ignored? Sex? Money? No-one will ever know because there will be no need for anything to be documented.