Imagine being the only known male relative of the guy on file in a case like this, and the SWAT bangs in your door. Except that you have a half brother you didn’t know about - nobody knows about him including your dad. DNA will eventually clear you but in the meanwhile you’re in for some considerable unpleasantness.
You’re right I hadn’t thought of that. You could also look at this the other way. What if Mr Perps dad got around so there are now several children that you would have to seize their DNA in order to find out which one actually did the crime. That would be illegal search and seizure in my book on those sons that were not guilty of any crime.