There's a particularly bad argument I keep saying places. It has this form:
1. Some religious people do action x.
2. If (1), then non-religious people (like us heathens) are justified in doing some corresponding action y.
3. Therefore, non-religious people are justified in doing some corresponding action y.
I'll give an example. I saw a facebook discussion recently about wearing atheist t-shirts, and a bunch of people said that if religious people can wear shirts to show off their religion then we can wear shirts as atheists to show off our irreligion.
It's true that we have the right to wear atheist t-shirts. I wear one sometimes. BUT it's a really crappy argument to just say that we should because some religious people do. Consider this argument:
1*. Some religious people kill apostates.
2*. If (1) then non-religious people are justified in killing people who convert to a religion (i.e. the corresponding action to killing apostates.)
3*. Therefore, non-religious people are justified in killing people who convert to a religion.
That's non-sense; no sane atheist would sincerely think that premise (2*) is true. This is something that matters for arguments about atheist billboards, bus signs, t-shirts, and so on. It's not because religious people do those things that we should -- one *can* provide other kinds of arguments, but obviously we shouldn't just mirror what religious fundamentalists do (in fact, arguing that we should likely drives the atheists-are-just-as-fundamentalist-as-religious-fundamentalists meme.)
