This is the matter of economic expediency.
The full refund policy is usually offered by large chain stores. I haven't seen many small retailers offering the same. Have too many people take advantage of policy and you may be out of business before you know it.
Large chains have dedicated teams that use statistical data and measure the projected cost and benefit of one or another action.
Everything in such operations is thoroughly calculated, ahead of the time (and they make appropriate adjustments if circumstances change. For instance, Barnes and Noble has 30 days limit on returns now, as opposed to indefinite/limitless return policy they had in place years back and etc.).
The loss incurred is considered a "cost of doing the business" and often factored into retail price that everyone pays.
The benefit of the policy is the Marketing tool, which allows to instill high degree of confidence in mass buyer which results in greater volume of purchases (to the point that costs are justified).
Also, due to economic reasons, fewer people take advantage of the policy than you may think. Even if you buy it and later feel remorse, you may discover it's not worth your time and effort to go back to store and return it, or ship it, but better to just forget about it.
It's purely business related and justified policy, not so much about perceived ethics of mass buyers.