This has little to do with defective products. People just expect to be able to return any product they don't like, or if they got the wrong one, or for whatever arbitrary reason. I would always be asked by customers if they could return it if they didn't like it, and they were always dismayed to learn that we generally only took returns if the product was defective. I lost more than a few sales that way.
My guess is that this comes from two things. First, most people are wholly uninterested on doing any product research. They want to just buy something that seems right, and then just take it back if they don't like it. Second, some stores will accept returns so readily (sometimes even of merchandise not actually sold by that chain!) that customers now expect it from everyone. Khol's is particularly notorious for this - we joke that their return policy is "yes."
The rules on how long your product has to work are better, though.
I love the Ikea pieces we have. As someone else said, they're far better than other put-it-together-yourself pieces I've ever bought. However, quality has been flaky. I bought 4x of a shelving unit (Trofast), and two of them had a single piece where the holes were not aligned correctly. (The first one, I thought maybe it was just me, but it was clear after putting together two more that #1 and #4 were defective.) Fortunately, it was a different piece in each, so I was able to cobble together three pieces of furniture and return the fourth.
They were very gracious and allowed me to return it despite the defect being only something you could tell when you actually tried to put it together. Since I was there with my kids, I would have been extremely unhappy had they asked me to demonstrate its failure. ;)
Judging from the wording of tormeh's comment, and if the regulations and customs are similar where s/he is located to those here in Sweden, unopened and unused was meant to refer to cases where you actually CAN return stuff no questions asked, such as when shopping through mail order or online (sometimes stores give you this opportunity anyway, but it's not something that's required by law or ubiquitous). It's not a given that you can always buy something, try it out and then return it if it's not explicitly stated.
If you're returning something because it was obviously defective when you bought it, then naturally it doesn't matter if you've opened the box.
I buy on impulse and I keep almost everything I buy. If you erect barriers, even psychological ones, I'm much less likely to return. These barriers aren't on Amazon. If I decide after receiving that it's not what I wanted I return it and buy something else.
That shop eventually went bankrupt and shutdown. It couldn't compete and win against it's current rivals and new online retailers. Who have same day delivery and simple return policy.
1. People would bring in furniture that was no longer sold at IKEA but claimed they just bought it 30 days ago.
2. People would go around to garage/yard sales and buy used IKEA furniture and then try to return it.