There are a number of issues here.
First - your innocent until PROVEN guilty. This means that without probably cause, you shouldn't be arrested in the first place. Yet such arrests happen. So to say that "if you ended up in the pokie, you must have done something wrong" is not true at all. I have been arrested and charged with a felony. Spent three days in jail. Got bailed out, ultimately went to court, and was acquitted because I had done nothing wrong. *All I will say is psychotic ex-wife*
Yet with this law, my DNA would be on file with an arm of the government, and they have no right to have it. So we have a problem already.
Next you have unreasonable search and seizure. DNA is by definition, the building block of life. Therefore, my DNA is part of ME! This law would allow a police officer to take part of me, against my will, for testing. The difference between a flake of skin or a strand of hair vs my finger or an arm, are simply matters of scale. So this would make it ok to take part of me apart, as long as it was a "tiny" amount. Bullcrap. Thats like saying if you steal a million dollars its theft, but if you steal one dollar from my wallet its ok because it didn't hurt.
This could easily be gotten around anyway since DNA can be collected in ways that do not violate either this or the next point.
Right to privacy - whats mine is mine - and my DNA is mine. Now there are times when that right can be overridden, but they require a warrant signed by a judge. Whats wrong with having to go to a judge and say "we need this, here is the reason we think we are within the law to get it" and have him decide? This violates the whole idea of due process.
Whats amazing is the last two objections don't even need to be there. Because say your suspect is a smoker.... watch him take a smoke break at work, then when he flips the cig, go reclaim it. Sure its disgusting, but by discarding it, he put it in the public domain, so you don't need a warrant to procure and test it. Get a match? Then go to a judge and say look, we got a suspect, watched him smoke a coffin nail, retrieved it and had it tested. It matches - so we need an arrest warrant.
The above actually occured in a case BTW, and the DNA evidence was accepted as having legal standing despite defense objections. The conviction was also upheld in all following appeals.
It doesn't have to be a cig, can be a cup of water, a soda can, a fork, etc. You get the picture.
Final problem..... DNA is NOT the "end all" of identification. It is nowhere NEAR the 99.9% "certainty" often quoted by law enforcement.
Read here and see for yourself:
http://blog.simplejustice.us/2008/07/20/dna-what-are-the-odds.aspx
Many here know how statistics can be manipulated, but the fact that law enforcement will not allow the DB to be checked to determine the true "odds", and instead bases the number off their "estimates", tells me quite a bit.
Simple solution - DNA on file at conviction, or follow due process as is required now.