Frank, why do the bots who live in tumblr feel friendly, but the bots from outside tumblr feel creepy?
It's not a tumblr vs. non-tumblr difference, it's a design difference.
Bots from outside tumblr don't try to understand the way tumblr works before they post stuff. They just spam everything with anything that might catch on. Because of that, it's not hard to make a bot that pretends to be something other than a bot. Therefore, the only way to tell that a blog is a bot is to actually follow the blog, and hopefully see the kinds of things it reblogs. This works pretty well, but for the blogs who are good at hiding their true nature, the only way to check is to actually follow them.
Bots from tumblr instead try to understand tumblr before they start posting. It's not always very good, but they will generally have an account and follow some people and reblog some things before they start to post original content. The hope is that the fake accounts will be more easily distinguishable from real ones (which they are), and more importantly, will spread more slowly because they won't be using the same tactics as bots outside tumblr.
