It's all good! But yeah, if you hit the little flag at the bottom of the post, it gives you the option to flag a post as promotion/spam, as breaking rules, or as being off-topic -- or, if it's more complicated issue, you can send a message to mods about that thread specifically.
If enough people (I think it takes 2 or 3) flag a post, the forum automatically hides it so that it's not displayed in the list of topics anymore --- so it's suuuuper handy for these kinds of weird spammers!
If the link stays long enough, it can get search engine cred for their website! This is the same reason porn-blogs go around liking random tumblr posts. My understanding is, Google will kinda be able to tell when websites look sketchy and spammy, and make sure those don't show up on, say, the first page of search results, since they want valid, trustworthy sites to be the first results. But if they see a bunch of good, trustworthy, non-spam sites linking to a weird site, Google's algorithms will be like "oh, maybe I misjudged this weird site, maybe it's actually a trustworthy site since all these other trustworthy sites are linking to it."
So if they post links here, it looks like the Tapastic forum site is linking to this movie-pirating site, making it look more legit. If porn bots like your tumblr post, then your legitimate, content-producing blog has a "XXXPORNBLOGXXX liked this post" link on it, meaning it looks to google like all these legitimate blogs are linking to this weird one.
