I think it’s more to do with the fundamental structure of the site. People on Tumblr want notice too (it’s a microblogging site), but the way it works is so fundamentally different from most other sites. First, while there is a ‘you might like...’ recommended post function, it’s really minor and doesn’t have much of an effect; in places like twitter, massive engagement is necessary to gain notice, so if people post, they want interaction that drives *their* engagement metrics, rather than having it ‘stolen’ my a retweeter, and they assume that Tumblr tags and reblogs work the same way. They don’t. They absolutely don’t.
On Tumblr, a post getting noticed is more of a distributed effort. Primarily, people engage with posts on the blogs they follow; engagement metrics are completely meaningless except in the sense that more reblogs mean the post is on more blogs (and therefore has a bigger audience). Replies and likes are like nice little communications with friends, but people who do want others to see their posts care primarily about one thing -- reblogs. On Tumblr, unless the message is clearly meant to be private (like someone venting about a bad day), the nicest thing you can do is reblog. If you want credit, you want reblogs.
Tags are a bit more subtle, but the way we use them is also born out of Tumblr’s structure. People started talking in the tags because it was the only way to hold a conversation on a post that wasn’t permanent. If you add a comment to a post, it’s part of the reblog chain, and back when this site still had that stupid line quote layout that rendered posts unreadable after a handful of comments, it was pretty fucking rude to make a post less readable just to say “This!!” or “I saw this same thing at the supermarket” or something. Posts are more readable now, but post length is still an issue; long posts are considered a pain to some users and not read as often, so people don’t want to reblog with comments, as the site is designed to be used, unless they have something to say that’s a) worth the clutter and b) that they want people outside their immediate followers to see. Tags are comments to your followers; adding onto a reblog is a permanent addition. So if people want to make a little joke to their followers, or a personal comment, it goes in the tags; if they want to add to the conversation in an enduring way, it’s added to the post in a reblog.
(Tumblr has a post comment function, but very few people use it unless they want to say something specifically to the OP, because it’s shit. Tags and reblogs serve the intended function of post comments far better than post comments do.)
So that’s the cultural base, and that leads directly to the ‘peer review’ that above posters have mentioned. When people see hilarious tags, of course they want to add them to the post, either to respond or so that others can see them. This means that people who want to add something permanently to the post, but who want to make it look like it was intended as an aside to followers or who don’t want to look arrogant/intrusive about it, can put it in the tags and hope that a follower finds it funny enough to ‘how dare you leave this in the tags’ them. (There’s nothing wrong or underhanded about this, it’s perfectly normal social communication.) The thing about this particular method of addition is, it removes the identity of the original tag poster. In some circles it seems polite to @ the tagger to ID them, or to be like “tags from X”, but this isn’t the norm; it’s usually accepted that if someone adds your tags on, you’re anonymous unless you respond and ID yourself by implication. To a twitter user, this looks like engagement theft. To a Tumblr user, it’s a cultural norm.
Keeping someone’s tags when you reblog is just a temporary/private version of adding them to the main post. Saying ‘I found this funny enough to keep but I also don’t want to clutter the post itself’. This is why it’s considered normal here, but outsiders might find it rude -- we’re used to making concessions for usability on Tumblr and accept that this is a way to pass on something funny to a small group, whereas users of other sites are still thinking in terms of engagement metrics leading directly to them in ways that don’t apply here.
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