Interesting ... we have had this discussion before, but this time people seem a lot more concerned with ethics than last time ... glad to see that. In the past, there were too many people who said that as long as REI is willing, they saw no problem.
I even saw (occasionally) advice that the way to sort out a pair of boots you were uncertain about was to buy them from REI, take them on a backpack, and then return them if you did not like them. Keep on until you find a pair you do like. I could not believe people would openly admit to doing things like that!
Personally, I feel that abusing the policy has various repercussions, none of them favorable for the rest of us. To me, the ethical policy is simple:
* If the item is in *new* condition -- i.e. if REI can put it back on the shelf and sell it as new -- then go ahead and return it.
* If the item was defective, or was misrepresented (not just that you misunderstood), then go ahead and return it.
But it is *not* a free rental or free trial service! It is not ethical to by a pair of boots, take them on a backpack, and then decide you don't like them and return them.