An add-on is an entirely different situation to a build. The idea of the game (and the reason we have the ability to save anyone's bricks on any server) is so that people can find something they like, take it away and then play with it or improve it.
This only becomes an issue worth mentioning when the person specifically aims to trick people into thinking that he made the build in question. If he's just saying "here's a pre-existing build I modified to look like something from a TV show" then it truly is a worry when it kicks off a stuff-storm of this magnitude.
Actually, it isn't quite that different. When you host a build, you risk someone coming in, seeing it, liking it, saving it, and playing with it later. Now, mind you, that is not the bad part. I do that from time to time, mostly if I like the build but don't like server. It's the same thing when you publicise an add-on. You run the risk of someone seeing it, liking it, saving it, and playing with it later.
It's just that with builds, it's much easier to edit them. You take a build and then load it in single player or on your own server. Then you can edit it however you want, by placing more bricks, painting preexisting ones, changing events, and so on. If you show it off in the way stated before, then you are effectively "stealing" it.
Add-ons, it requires a bit more thought on how to edit it, but it can be done. If you edit it for yourself, just for your own amusement or to improve your server, you don't necessarily have to give credit to the original creator, but it would be nice to say who "helped" you to whoever asks. If you show off your edit of the add-on (for example, creating a new topic about it) and you do as before, it's just as bad.
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