Why did God let people think demons cause epilepsy?

[Razzle]

@ Razzle: Yes, you did say all and always in the OP.

No, read it again and note the context in which the word "always" appears. (Also note that "all manner of" does not mean "all".) I didn't say the Bible says all diseases are always caused by demons. I said it neglects to point out that certain diseases are not always caused by them, which is a highly misleading omission for the creator of the universe to make in a book that would be read by thousands of years' worth of highly uneducated and irrational people.

While modern Westerners rarely interpret the Bible as saying all such symptoms are always caused by possession, that's only because we already know before we read it that they are usually improved or even eliminated by medication, which would not be the case if demons were in control of the brain. It's common knowledge to us, so it's very easy to interpret the passages that you point to as meaning what you think they mean.

But think of people who don't have the benefit of that knowledge, as most of humanity has not since the time of Jesus. To them, having the creator of the universe come to Earth in person and inspire/dictate a holy book for us to read, giving examples of diseases such as epilepsy and psychosis being caused by demons without any examples or mention of any other causes, has often lead to them assuming that there are no other causes. The passages you cite are too open to interpretation to prevent that from happening, which is proved by the fact that it has happened and does happen throughout history and in uneducated parts of the world today.

The first one you present, Matthew 4 ("People brought to him all who suffered with various illnesses and afflictions, those who had seizures, paralytics, and those possessed by demons, and he healed them"), comes the closest to being an adequate rebuttal. It does at least seem to suggest that one of the conditions I'm talking about, epilepsy, is not always caused by possession, by listing them separately. However, it's not the sort of thing you'd necessarily notice unless looking for evidence about exactly that question, which you're not going to do unless you already have at least an inkling that epilepsy is not always caused by possession and are interested in the issue.

If as an uneducated villager, you want to know about demon possession, you look to the passages dedicated to it, like those labelled "Jesus heals a demon-possessed man". Those would be good places for God to place his medical disclaimers. You probably don't scour the New Testament for any casual reference and analyse the construction of sentences in your translation to work out whether epilepsy and possession are synonymous, unless you already have reason to question that specific issue, which most people throughout history have not. That line is not good enough to prevent exorcisms of epileptics - if it were, we simply wouldn't have epileptics being exorcised by Christians, ever. The even bigger problem with it as a response to this criticism, is that it only specifically mentions epilepsy, out of all the conditions that get mistaken for possession. It does absolutely nothing for schizophrenics or others acting in ways similar to the man healed in Mark 5. 

Your other two passages merely suggest that not all disease in general is caused by possession, and I never claimed that all diseases are victims of this phenomenon anyway - only the specific kinds of diseases and symptoms that are associated with demons by the Bible.