I'm not an expert, so this is just a guess. What I know is that Stumbleupon doesn't actually send you real visitors in most cases. Instead, they fetch your page from your server and display it in a frame on their site. The viewers have to click "visit site" to actually come to your site, but most of them don't do this. So the re-direct may not go into effect in most cases. Anyway, what traffic most sites get from stumbleupon is poorly targeted so isn't worth as much as traffic from the SERPs or from relevant backlinks.