It is unacceptable because, at least with most stores, people have every opportunity in the store to try on clothing and to decide if they like it. The fact that people have buyer's remorse or decide they do not like the product in no way suggests a failure of the store to deliver the promised quality of merchandise. In other words, buyer's remorse shouldn't result in a store losing a sale.
I think if the tags are on and it is in resalable condition, there's no harm in returning something just because when you got home and had more time to try the item on, you decided it didn't work for you after all. Stores want you to do this, because in the end knowing you can return something leads to more sales.