By all and sundry. Including you.
With due process denied, how does a person manage to prove that they are a citizen? If their right to aquire the necessary information to prove this important fact is infringed (quick, do you have proof of your citizenship in your pockets now?), how will they ever manage to (having essentially no rights at all) prove this?
In fact your average investigator, if they realize that they did infringe on an American's rights so badly that they were going to be up shit creek without a paddle, would just simply engage in CYA and keep the case from ever getting out. I know this, it is obvious, and if you stop playing Devil's advocate, it will be obvious to you as well.
Therefore no matter what the law says about its only applying to non-citizens, in effect it can be used on anyone, citizen or not, who attracts the wrong kind of attention from the State.
But then again, no matter how much I hate it, this isn't really much of a worsening of the situation in the US. Did you know that there exists a branch of the US government who can put up roadblocks between you and work, can haul you out of your car, and put you in jail for an indefinite period of time with no ability to see a lawyer or communicate with anyone?
Well there does.
The branch is the Border Patrol, and they can do this within 100 miles of any US border, the coast included. (The ocean is less than 100 miles from your house? Right?) They only need to convince themselves that you probably are not a citizen, and voila! You can forget everything in the Constitution! I kid you not.
In fact if you look at the dragnet after 9/11, hundreds of people have been in jail for months now. We are told that all of them are illegal aliens. Would you care to bet that they are? That no mistakes were made? I wouldn't. All that Ashcroft is doing is making this loophole worse...
Cheers,
Ben