That is some flawed logic right there. You cannot compare legal battles to getting your video game work stolen. Honestly, at this point, I'm not trying to use real life instances and you probably shouldn't either with these examples your coming up with. I can't tell you what to do though.
Posting it and having evidence that it's yours is basically protection from theft.
No, you can't, but it doesn't make this any more right. People don't profit from dream towns and it's ultimately just a game, but it's also a hobby that people put lots of time and effort into. I've said before in this thread that I don't have a dream town and this personally doesn't affect me, but I can at least try to understand why some people wouldn't want their towns ripped off square by square.
According to the logic you're using here, people shouldn't share their towns unless they're blogging about it a lot or something to have "proof" and otherwise they're not protected from theft. That's more or less true, but people should also be able to enjoy a feature of the game to share the things they've made with their friends and the community without having to worry about this. I visit dream towns a fair bit when I'm bored in the game and I like visiting the dozens of really well done towns that I've found. It would pretty sad if we couldn't visit any because people just wouldn't share them so they won't be wasting their time on something that someone else will take credit for instead.