Out of deference to the Holocaust, the US government doesn't keep demographic information on religion. As you say, the theory goes that a record of who belongs to what religion might be used to commit genocide.
But this theory isn't applied to anything else. The same government collects detailed information on everyone's race, despite the fact that race is a much more common target than religion is.
Surely you're aware of at least one genocide that was greatly aided by detailed demographic data?