Why do theist often drop the letter s when referring to atheists?

[Mister Agenda]

For some it's a lack of education

Others do it just to rile people up

How do you figure that it is a lack of education?   Do you mean that they don't realize they are doing it, and need to enlightened to the issue?

The charitable explanation for someone who does it routinely is that they don't know any better. Is it your position that they're doing it on purpose? Because that's a different kettle of fish.