Why do people think you are anti-muslim just because you don't support ISIS?

I don't know. The two are, really, very different subjects.

ISIL are the scumbag, freakshow far right extremists, and have very little to do, really, with mainstream Islam.

I have never met anyone who so thought.

The word 'radical' means to get back to the roots of something. The roots of Islam is Mohammed, and there is plenty there to justify violence. 'Radical' Muslims (all professing members of the terrorist group Islamic State - ISIS), claim that non-radical Muslims are NOT really Muslims... and they are right.

In the same way I wish all professing Christians would be radical and get back to their roots, which is Jesus.

it happens most commonly when people make sweeping, anti-muslim comments.

Why do people think you are pro-muslim just because you support freedom of religion?

No one supports ISIS, except perhaps Saudi.

Islam is false and must end

Don't worry, nobody thinks that.