It's not unethical, because it's their own policy.
They have that policy in place, for the fact that it will increase their sales, because some people just need the psychological reassurance before they purchase an item, that if they don't like it, they are able to return it. And a lot of these people who need this reassurance and end up not liking the item as much as they thought; won't bother returning it anyway because it's an effort. So for many companies, it's worth them having a policy like this in place to increase their sales.
The unethical bit usually comes from the company, who want to create a guarantee and then not honour it.
Here in the UK I remember Tesco had a policy that if you were overcharged for the item, you could get a full refund for that item and keep the item. Now this happened to mostly small things (food), and rarely big things. But a popular online forum would post all the misprices, and people would go into the store to buy those item. After a few years, Tesco changed it's policy because people were "abusing" it. Excuse me Tesco, the only way people could "abuse" such a policy is if you were constantly overcharging people at checkout, for items they thought cost what you displayed on your shelves. You can't abuse a policy with the occasional misprice. But if Tesco is regularly showing lower prices on the shelves, then charging you more at checkout, then it's not the customer abusing the policy, it's the supermarket abusing the customer. And they get away with it because most people, buy a lot of items in one go, and by the time they reach checkout, they can't remember the prices that were advertised on the shelves, and so millions of people are getting overcharged for God knows how much, every week. I guess every little helps (Tesco's slogan).
So go for it!