Horror movies are aimed at a certain demographic: teenagers to early 20's group.
So according to the wisdom of the writers and producers of many schlock movies such as The Amityville Horror remake et al, as far as they're concerned nobody from the aforementioned demographic will actually chase up or research the so-called facts in which the "remake" are based on. Why not? Because they want to be entertained.
As far as they (the writers and producers) are concerned, the demographic I have mentioned will believe that if it says "based on a true story" then it might as well be 100% accurate, having little respect or concern for the audience who pay good money to watch this crap!
"May the forces of evil become confused on the way to your house."
- George Carlin. Comedian. (1937-2008)