The most popular book clubs all use a negative response model, where when you sign up you get a certain number of books at a great price, and agree to buy a certain number of additional books at the regular club price.
They send you a monthly list of books available that month, with one marked as that month's featured selection.
If you do nothing, you automatically receive the featured selection and are charged for it at the regular club price. If you do not want the featured selection, you tell them via a return card or their website.
An example is the Science Fiction Book Club [1]. The front page has a link to a "how it works" page [2]. Note that the "how it works" page doesn't actually tell you about the negative response aspect. It tells you to read the membership agreement for complete details and links to the agreement [3]. It is in there that you get the details of automatically receiving the featured selection.
(Things are similar for the Book of the Month Club, the Scientific American Book Club, and a whole bunch of others--because they are all actually run by the same company, and are using the same template for their web sites. The Columbia House DVD Club too).
Compare this to the JustFab page. That too has a "how it works" page [4], which is linked to from the front page. That page tells you about the negative response part.
The front page of the book club does say, when touting the initial book offer, that it is "with membership", so it is clear on the front page that you are going to probably have to sign up for some kind of membership to get that deal. The JustFab page does not make it clear that you must become a member to purchase.
With the book club, if you fail to make your negative response, you get a book not of your choosing. With JustFab, you get a credit that you can use on an item of your choosing. My guess is that the vast majority of JustFab's customers buy several times a year, and so they are able to fairly quickly put the credit to use.
So why does JustFab draw so much more fire, when the seem ostensibly quite similar to the book club? I wonder if the fact that their subscription if for credits makes a big difference? The book club pushes a featured selection each month, presumably something they have made a volume deal for in order to get a good price. For this to work, they really need their featured selections to sell well. With JustFab, the credit is generic. If they need to push some specific item in order to support a good price for it, selling credits does not help. Would people find JustFab more acceptable if instead of a credit, they actually sent you an item once a month?
[2] http://www.sfbc.com/howitworks
