It feels like it's set up to make returning substantial numbers of pieces of clothing more normal and expected. I'm guessing at other retailers with a free shipping/return policy, they'd frown on very high return rates.
My wife loves them because the stores near us don't have a huge selection of Petite sizes, and her return rate is somewhere around the 30% range. The experience is so good that she's incredibly loyal to them.
* Boohoo
* Pretty Little Thing
* Missguided
* Marks & Spencers (non-food)
* House of Fraser (under 20Kg)
* Debenhams
Outside of these, with "Collect Plus" now being next-to ubiquitous in the UK (over 6,000 participating stores) any store using Collect Plus (or DPDs Ship-to-shop) can offer free returns via the stores. As an example of how useful this is in general, the tiny corner shop on my street is a Collect Plus collection point.
They even have places to try on clothes, and packaging materials there, so you can return the items without ever taking them home.
Unfortunately they are quite expensive from a retailers perspective, or roughly equivalent to a Prime membership if the user subscribes directly (so they can use it with all online stores).