For the same reason it is ok to collect felon's fingerprints to compare to crime scenes...the felons have shown they commit crimes.
That's not a thing. It's innocent until proven guilty; it's legally impossible to gather evidence because they're guilty to use against them. Hell, innocent people's fingerprints can be, and often are, collected at crime scenes. The police can use innocent people's prints to establish a timeline or figure out what their role was.