This is a gray area. SUSPECTS can be requested (but not required). Persons who ARE arrested can be fingerprinted against their will. Persons who are NOT arrested do not have to allow themselves to be fingerprinted. No one has been arrested in this case yet. Those who gave prints, DNA (saliva swabs, etc) handwriting samples, etc. did so willingly. No one is COMPELLED to talk to the police- that is why people are read "Miranda" rights (aka you have the right to remain silent, etc.). Even someone who has been arrested does not have to talk to police. If they are on trial and are called as a witness they can STILL refuse by invoking their 5th Amendment rights.
There has long been discussions about whether any of the young male children who were party guests at the White's gave samples. I doubt it. Even as adults now, they do not have to. Remember the "unknown male DNA" does not prove an AGE for a donor. Even DNA like semen only proves an age of at least puberty- the exact age of the semen donor cannot be determined- only that they are old enough to produce semen.
In this case, the "unknown male DNA" was skin cells- something that is easily transferred by a third party innocently- touch a doorknob, toilet handle, shake hands, touch your own clothes (as in using a toilet) and someone else's Touch DNA (skin cells) are likely to be on your hands and then on your clothes. Skin cell TDNA is different from DNA from a primary source like saliva. blood or semen. The presence of DNA from blood, semen on a dead child's clothing is MUCH less explainable innocently. And like ALL DNA- until and unless a donor is identified by name, useless as far as "clearing" anyone- especially clearing people who were present at the crime scene during the time the crime was committed.