Some sites (such as Ancestry) demand that the only samples that can be submitted are those of the member or of a named person who has given the member explicit permission to do so. So no police samples can be loaded without breaching the terms of service. Others (such as FamilyTreeDNA), on a case-by-case basis, grant law enforcement agencies and some third parties working with law enforcement permission to upload a genetic file of an unknown person. Those files can be matched against an "ordinary" files only if the owner of that file has given explicit permission for that to occur. The data aggregator site GEDmatch operates on a similar basis.
The default setting for an account on which permission is required for law enforcement access is NO. If you have want your DNA data to be available for use in this way then you must login, locate the switch in your account settings and change it.
On FTDNA: login > Your account > Manage Personal Information > Privacy & Sharing > Law Enforcement Matching (LEM)
On GEDmatch: login > User Profile > "Police badge" icon
In discussion, some members noted that although individuals may see "their" DNA as personal property, the fact that they share large amounts with other family members means that the decision made by one person on sharing could conflict with the wishes of close relatives.