> Why do this ?

Preventative measure.

> violate sacred principles

Algorithmic blacklisting is a standard of proof.

Some others are :

Reasonable suspicion
Probable cause for arrest
Beyond reasonable doubt

Any preventative ( rather than curative ) measure will work only if it operates before the fact, and to do so, it works to operate under a standard of proof that is less than that required for an after the fact measure, being less than required for a curative.

So a custodial sentence is a curative ( restrain offender from doing more harm ), after the fact, and after the facts of the offense have been proven beyond reasonable doubt.

Arrest and detain, or prevent from flying, is a preventative, before the fact, based on a standard of suspicion.

The balance here is that the inconvenience caused to a detainee ( or a blacklisted flyer ) is less grievous, is better, than the possibility of a harm caused by not detaining or preventing that suspect. This balance is guided by the sacred principle of harm minimization ( protecting people ), and the preventative measure is justified.

Innocence until proven guilt does not make regards to suspicion. A person can be innocent ( and is by definition ) until they are proven guilty, and at the same time they can still be a suspect. Innocence until guilt does not equate with a person being beyond suspicion.

If suspicion was not a sacred principle, a lot of investigations would lead nowhere, and a lot of preventable harm, would occur. In fact, it's reasonable to argue that the courts often err on the side of devaluing suspicion, resulting in offenders who, once they are released and commit a crime, find themselves in the middle of a community backlash of people asking, "Why wasn't this person prevented from doing this?"

In the view of the courts, a reasonable suspicion, or a probable cause, or an algorithmically determined "black hand", is a workable balance in effecting harm minimization. Its carriage is the very due process of law, and by implication, a defense of the US constitution.

Another way of seeing this whole thing, is that any curative is by definition insufficient, by virtue of a crime having been committed, how can any punishment ever change that ? So even a curative, can be seen to be really a preventative on the possibility of future offenses carried out by that person.

I don't imagine Snowden would be troubled by this, as his whole crusade seemed to be founded on the idea, that if he releases the documents now, perhaps he can prevent the possibility of future abuses occurring, and create what he views as improvements. Snowden seems to believe more profoundly in the idea of a preventative measure even than the courts.

Finally, it's worth mentioning that algorithmic blacklisting is actually a step in the right direction. The lesser standards of proof ( such as reasonable suspicion and probable cause for arrest ) are by their nature and undertaking highly discretionary, and subjective, and also unscalable. An algorithmic means of administering assessments of suspicion, is something that you accept each time you pay the requested amount for your mortgage or use your credit card. The assessment of risk, carried out by machines and algorithms, is at least more fair and just, than if someone could deny your startup loan application, not because the algorithm said your idea was unlikely to work, but because the person simply didn't like you.

Algorithmic blacklisting provides the means to assess every person in the same manner, being the very definition of, "equality under the law."

To get reflexive, every time you find yourself suffering the inconvenience of being stopped at the airport for a random check, it works to remind yourself that this inconvenience is lesser than if you were dead, because someone hijacked your plane. You can't cure your way out of that, no one can. So while you champion that people are innocent until proven guilty, and deny the validity of suspicion, every day you judge people based on what you suspect them of being capable of. You don't let your child sit next to that man on the bus you think is strange looking, or you don't give money to that homeless girl, because, in each case, you make a judgement about what they person may do. You suspect your colleagues at work, you judge the people you just meet based on what they say, and you suspect your spouse, and yet when you find yourself in the position of being judged on suspicion, you claim the double standard for yourself, that the very thing you do, should not be done to you.

If you want to think clearly about things, it works to see clearly that the double standards you employ are a contradiction, and how useful suspicion is, both from a personal, and a legal, point of view.