"As NYU law professor Erin Murphy told the New Orleans Advocate regarding the Usry case, gathering DNA information is “a series of totally reasonable steps by law enforcement.” If you’re a cop trying to solve a crime, and you have DNA at your disposal, you’re going to want to use it to further your investigation."
If the police broke into people's houses to snoop through their private effects, that would help solve crimes too, but it doesn't mean it's "totally reasonable" -- why is snooping through private DNA records "totally reasonable"?
It comes down to the third-party doctrine[0], which is the same reason the text messages or emails you send are not subject to 4th Amendment protections. While in your home you have an expectation of privacy the courts have ruled that once you release something to a third-party this expectation is not valid.
If the police broke into people's houses to snoop through their private effects, that would help solve crimes too, but it doesn't mean it's "totally reasonable" -- why is snooping through private DNA records "totally reasonable"?