Technically, you're not actually snatching it, as their post remains the same on users feeds, the front page, their feed, or even when being viewed on other members "saves", etc. What you're doing is spreading it further by mirroring it into yet another tribe, and essentially helping the poster by giving their snap even more exposure which results in the post being listed higher on the front page, etc. By mirroring it, you change the look of the snap only for the tribe you're mirroring it into, and generally the changes will be contextually minor (maybe a different cover image, or a slightly edited title, or a longer intro).
So as you can see, the reward is getting much more exposure on the content they post because tribe chiefs and mods can now mirror any snap into their tribe, all while giving the user credit by having a link back to their profile, etc.
Awesome. Thanks for the explanation.