It's just semantics I think everybody knows what we mean. I guess it's easier to just say "religion" rather than- "belief system" or whatever. It's just a matter of getting butt hurt.
It's like squares and rectangles. A religion is a belief but a belief is not necessarily a religion. But an atheist is a square and a agnostic is a triangle and a baptist is a trapezoid at the end of the day who gives an ish?