I'm not a fan of it but, the way their system functions it really doesn't make sense to do returns for anything but defects. Being setup as a collective buying community where they place one order for all of the items once all the customers have signed on, they don't have a warehouse full of the items they sell that a 'did no like' return can go back to.
Stores have to absorb the financial impact of these type of returns and thus have to pass that expense on to their full customer base. Drop would have to raise their prices to cover the loss from these returns and the extra employee time that would be required for processing them as well as trying to sell them through some other means.
And that doesn't even begin to get into the customer service time required for interacting with the customers who forget to include things with the returns or return items scratched or dinged up. I'm sure most people here are like me in that when I do return something I make sure it is spotless and packaged exactly as it originally was. However, having processed customer returns before, I can tell you that a lot of people just cram everything in the box and expect a full refund. Most people's reaction to that problem is "then just don't refund for items that aren't perfect" But if they don't get the full refund because of something they did they will give you bad reviews and even fight it with their credit card company; it is difficult and possibly expensive for a vendor to fight a chargeback when the customer has proof of delivery. So by not accepting the 'changed my mind' returns they've removed a fair sized expense from their business model.