I feel like the angry priest who has discovered a witch -- they pervert the teachings and lead the faithful away from the truth. They are dangerous because they are so seductive with their lies and half truths, corrupting honest minds with heretical nonsense.
And even if that sounds like a ranting madman, even in sane review, we have to admit that they ARE heretics because they know how scientific method works -- you can propose anything, like "Aliens built the pyramids in Egypt and South America" but then you have to prove it.
To just say, "it would have been too hard for people to move those stones," isn't enough. That's just saying you don't know how they were built, and just because you can't figure it out doesn't mean aliens are involved.
This is where the heresy comes in. If someone comes up with a theory for how they moved the rocks, you are welcome to try to prove or disprove it, but not ignore it. If they actually do experiments and show the fact it could be done that way, you can't say, "Well, I still think I'm right" unless you have some evidence to prove your theory.
I call it "heresy" because they present their ideas as science and then ignore the basic rule of science -- evidence trumps theory. Heresy is when you know the rules but you choose to ignore them.
"But, Michael," you say, "Ddin't I hear you say you really like Stargate? You know, that movie and TV series based on the fact Aliens built the pyramids?"
Yes, I do. Even if it is a silly, campy (now defunct) series, it never pretended to be real (well, there was that episode where they were dealing with the TV show that was about the Stargate program...) Some of my favorite science fiction involves faster than light travel, time travel and parallel worlds -- all of which are speculated on in the real world, but none of which are possible with our current understanding of physics.
Life would be really boring without speculation and fantasy. Without dreaming about "What if?" we would never have new stuff and new ideas. Look at the orginal Star Trek -- a lot of what was pure fantasy in 1966 is real now: we've got doors that open automatically, we've got pocket communicators, we've got computers that talk, and we even have a body of scientific exploration into the idea of warping space for faster than light travel.
Even pure fantasy like, "What if it turned out the pyramids really WERE built by aliens?" isn't just fun, but interesting in, "How would people react?"
There's a great series of scifi/fantasy stories about Lord Darcy, a Royal Investigator in a parallel world where magic has been established as a scientific process, and physical sciences are obscure and arcane tools. I love those stories because they are internally consistent detective stories that you can figure out, even if the locked room mystery could be from a psychokinetic spell.
But fantasy and speculation aren't heresy. And, really, pseudoscience isn't heresy either -- it's delusional. Any immorality comes in when you try to profit from manipulating people with lies.
Don't get me started on TV mediums now THERE are some people who should be burned at the stake!