Must clay pottery be fired in a kiln for it to be "fit" for use in human consumption?

I've noticed that all the clay I look at in the craft shops, etc, is labeled as being unsuitable for use in human consumption, even though many are also labeled "non toxic" (???). Is there a special type of clay that is "fit" for use with food, or is it the kiln-firing process that makes it safe, or what? I mean, I don't think that indiginous peoples had access to kilns or glazes when they were using their pots (?)... so, I'm a little confused here...