A warrant is not needed to obtain a DNA sample as done here. ANY bodily fluid or other sample capable of testing that a person voluntarily discards or leaves around is perfectly admissible as long as no other illegal search was made in order to find the DNA.
For example, if a suspect goes to dinner and drinks from a glass, then the police can take latents from that glass once the suspect is through with it. However, the police cannot enter a private residence to obtain a latent from a glass without consent or a warrant.
Anything left in garbage can be seized legally, as the courts have repeatedly that discarded trash is not subject to the 4th ammendment or privacy laws.
This was an extremely creative way for the police to obtain a sample. The police did not seize anything, the guy provided it.
Scruit, they don't need blood, as saliva provides the DNA coding also.
Tricks like this are done all of the time, the media just usually do not get wind of it.
"Speed is fine, but accuracy is final" --Bill Jordan
Remember those who died, remember those who killed them.