The data are collected under one set of premises, but they get used for other purposes. (Barry Bonds's voluntarily given samples were supposed to have been destroyed, but they were later subpoenaed-- involuntary production of fluids, essentially).
Remember Kyllo: one of the things that made the search illegal was that the technology was not available to the public.
If we let our subjective expectations of privacy lapse thanks to Facebook (e.g., but also other private corps.), maybe we can't say we don't expect the government also to be sifting through the data.
There's always a tendency to use incidents as an excuse for a power grab. And once power is granted (as in USA PATRIOT), it's very hard to take it away again.
The court does make this a solid 1A issue, though: no beating around the bush there.
And note that not every adherent of any given faith complies with every doctrine identically in all situations.