Here is the deal – if the system is abused – it will be changed to prevent such actions in the future. Which may come as a removal or the "return anytime warranty", reduction of the dividend or other measures.
The same thing happened with Costco when people were buying flat screen TVs, computers and cameras, only to return them 2 years later (when those items became obsolete in technological terms) and used the full refund to buy new electronics. This is the reason why Costco now has 90 day exchange/return policy on all major electronics.
Now, what i can also tell you is that REI is allowing this use of coupons KNOWINGLY. As someone who works with computers and supports corporate networks with web-stores,I can assure you that these things are well known to the management. And if you can use the coupon for two separate purchases – it is because a decision was made to allow for such behavior. Programmers can disable this "bug/feature" at any moment. In fact, they are receiving real time statistics on orders, profits, number of sold items and coupon use.
If you have some doubt – call them and ask if it is OK to use coupon multiple times. This should resolve any ethics questions.

