How can anyone call this 'Horror'?

I agree with you. It is truly amazing how "genres" are continually misused. I go to the video/dvd rental place and spend an hour looking for something because it is miscategorized. Just because a film is weird, doesn't make it horror.
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Movies, especially these days, are not easily defined in one genre. They just don't fit neatly in one specific category. Look at Netflix and all the subgenres they have.

For me, I don't think Horror necessarily has to have elements of the supernatural or paranormal. Here is a good website I came across while looking at this: http://www.filmsite.org/horrorfilms.html

I hate IMDb sometimes! They are never any when you need it.

I group one as Gore (Inside, Cannibal Holocaust, Hostel) and the other as Horror (supernatural and fear of unknown/creepy). I think Gore needs to put as a genre as a lot of those types are being mistaken for horror.

I completely agree with you OP.

I would classify Eden Lake as a thriller, but I think it does have horror components even if it doesn't have paranormal or supernatural elements.

I'd classify it as horror based on the intensity and overall darkness and evil of the film.

This is most definitely NOT a crime thriller, a crime thriller is something along the lines of Silence of the lamps, where the cops were involved, even though the kids did create a crime, that doesn't make it a crime movie.
I would say this is a horror movie, because its the "cat & mouse" game played in the woods, also it had lots of blood, it takes place mostly in the dark, & the bad guys don't need a hocky mask, they are just pure evil.
Same goes for INSIDE, Funny Games, Ils, Just because there are no monsters, that doesn't mean it falls into the very first category based on the characters actions.
Thats like saying Paranormal Activity is a crime thriller because it has cops.
A horror film is something unsetting that strives to elicit emotions as fears, disgust or even startles the viewers

midnight muse w/ respect to your question i think you're one of those people who tends to think so much that they over look the basic elements. Well, this is what I think at least, it's labeled 'Horror' (which I agree) because a whole lot of horrible things happened through out of it and the ending was horrible despite crime is what brought it on through a process that's thrilling.

Well I just watched it and i have Verizon's Dish service... and they had it listed as Sci-Fi. Yeah.

So what I'm basically saying is, if a film is about some masked killer it will be horror, if the killer isn't masked, then it's a thriller, no matter how shocking or violent the film is.