Because they can. Civilisation spent centuries building up rights and due process, and then the authorities suddenly decided none of it mattered in this specific context because we let them. We should be livid. It's an absolute disgrace and an embarrassment to post-enlightenment humanity.
I can't use the visa waiver program to travel to the US because I was arrested once. Not convicted of any crime, mind you, just arrested. In the rest of society it's a pretty strong principle that guilt is decided in courts, not by police officers. No matter: anything that makes you seem less than the lowest possible risk is enough to deny you something. No presumption of innocence, no visa waiver program for me.
My girlfriend's mother has applied for a family visa that would allow her to immigrate to my country (where her daughter lives) permanently. The application takes several years. In the meantime she applied for a tourist visa to visit us for Christmas later this year. It was denied. We can only speculate why, but of course I suspect that since she has demonstrated a desire to immigrate permanently, the authorities consider her at risk of overstaying the visa. A mother who has done nothing wrong can't visit her daughter for Christmas because of this, and it makes me furious. She has no intention of overstaying: if this is why they rejected the visa it is again an assumption of guilt instead of one of innocence.
I wonder if there is any way to make it an election issue in any country. Parties seem to be unanimous on the topic, and most people don't travel, so it's probably not much of a pull for votes. Influential people travel more though.