Ah, just my skeptical nature getting the getting the better of me, I guess. I see your point, but I'm not 100% sure "there is no difference." In one case, a free sample of a product is offered, and in the other a bounty is offered for testing a service. Again, I see your argument and tend to agree in the big picture they are similar, and in most cased there wouldn't be anything nefarious in the offer.
However, to play devil's advocate I'll ask; what's an appropriate bounty? If one were to offer a bounty of 0.5BTC for testing a service, how unbiased will the reviewer be? At what point does it start to look like something nefarious?
I'd say everything that is close to a decent amount regarding time spent.
After this everyone has to decide for himself how much his time is worth.
I value my time at around $30..$100/hour depending on the technicality/difficulty of the task.
Of course to follow up on the 0.5
BTC you are mentioning, this kind of amount would raise a huge red flag (unless it involves days long of testing...).
Offering a free copy is no different then offering a bounty as long as the free copy does have a market value, which it has since the editor is managing to sell some.
If the item has a value FOR the tester is another story.