I know what you mean...it's bizarre, but that's the way it is. If I buy a DVD from a manufacturer with a sleeve, case etc, it's considered fair game to sell it on after you're done with it. If it's a DVD you have legally purchased via download-to-own/burn, then I believe some people might consider it akin to piracy (even though it's the same transaction/product, in effect).

It's no more or less damaging to the producer than when I buy a mainstream DVD secondhand off of Ebay or Amazon.

The key difference is that with a finished product such as a DVD, there is a finite number of copies in circulation...this will determine the availability of said item, and also the price on the secondary market. Piracy has the same effect as counterfeit currency in that it lowers the value of the item in question by making it less rare and exclusive.