Ah yes you're right, this sounds like a bug. We did prevent the granting of Contribute to The Public for a while, but this clashes with Surveys.

I have now enabled it to be set, so you can try again and hopefully all will be well.

There was however good reason for preventing this - e.g., if you allow public contribute to forums or photo albums they'll fill up with spam pretty quickly!

So, anyway, while you can now set your survey correctly, this needs a bit more thought before keeping this change permanently!

Thanks

Joe