I’m grudgingly holding to the “rag doll” principle for this question: if investigators can get what they need from your uncooperative (though unresisting) arrested body by simple physical manipulation, it’s legal (albeit very close to the line). They can’t take anything from you (DNA) nor compel you to communicate (right to remain silent), but can photograph, measure, etc. If they can take your fingerprints, they can feed ‘em to a computer and see if it will respond usefully.
In computer security, there is a difference between keys and identification. Fingerprints are identification. Passwords and encryption codes are keys. Both are needed for proper security; don’t rely on just one.
I don't know how accurate fingerprint readers can be, but I know that the ones in our office require you to put down your finger perfectly square on the reader and that you remain absolutely still for the time it takes for the reader to work.
If these are the same, I don't see any way that they could ever get an accurate reading if you just twitch your finger every time they place it on the reader. Even holding your fingers down in place, they would have a tough time to keep you from voiding any potential read on the fingerprint. Smashing your finger on the reader causes the fingerprint to squash and also become unreadable ... take it from me, I've done it by mistake and voided the read.