Such gradual expansions of programs appear innocent from up close and always present legitimate claims of practicality, but they create channels for illegitimate power awaiting application:

Law enforcement officials are vastly expanding their collection of DNA to include millions more people who have been arrested or detained but not yet convicted. The move, intended to help solve more crimes, is raising concerns about the privacy of petty offenders and people who are presumed innocent.
Until now, the federal government genetically tracked only convicts. But starting this month, the FBI will join 15 states that collect DNA samples from those awaiting trial and will also collect DNA from immigrants who have been detained – the vanguard of a growing class of genetic registrants.

Ironically, perhaps, the example of “a man found guilty of loitering for the purpose of prostitution” indicates a direction in which this program could head. If, say, the federal government begins attributing terroristic intent to gatherings of citizens with particular views, and if the action that triggers DNA collection is mere arrest or detainment, a broadly interconnected police and security system could begin amassing DNA information about a targeted political minority.
Approaching the matter from another direction reveals another spectrum of concerns. Who’s to say where DNA-related technology will go? Imagine that an individual targeted by the law is known to have a genetic susceptibility to a chronic health problem; investigators would thereby have a means of narrowing their search. Suppose technology advances to the extent that DNA can be collected casually and analyzed on the spot, or that there proves to be genetic correlation for certain beliefs.
Speculation in this line rapidly moves toward science fiction, but so, too, does reality. DNA is the key to our biological constitution, and when it’s clear how knowledge of it may be used as a tool for oppression, it’s likely to be too late.