So if you think that Easter worshippers in a church are by definition Christian then what's the issue?
Obama: it's sad that 10 squares will killed this Saturday.
Conservative: Fuckin' coward doesn't want to say that rectangles were murdered.
Why are they(not only Clinton and Obama) using this strange term instead of the much more well known term?
I think they did that because they are afraid, many far right figures use islamist terror attacks against Christians as justification for their anti muslim agenda and they don't want to feed into that. But I think hiding behind ambiguous words is the wrong responds.
Perhaps, or maybe they said Easter worshippers because the attack happened on Easter. I disagree with the idea that the term is more ambiguous because like you said the only people who would be worshipping in a church on Easter would be Christians.