Your "stealing others work" is someone else's "forking a project to add missing functionality".
If you are going to write a plugin and expect the community to build their sites upon it, and depend on your plugin's functionality, then I think you should realize that if you stop developing it, or worse pull the plugin from the site in a huff, that is very disruptive to the community. I know I've personally reduced my consumption of vB.org plugins because of authors "taking their ball and going home". I don't want to find myself dependent upon a plugin which will disappear in 3 months.
Now if someone is taking plugins and repackaging them as their own without credit to the original plugin, and not putting any new features, then I can understand the frustration.
But if you releases a free plugin, stop developing it, and other people see that it would benefit from new features or further development, I don't think you should be shocked if someone else wants to continue your work (with appropriate credit and backlink). The spirit of the modification community is adding on to existing code, whether that's the base forum, or other people's plugins.
I think when you upload a plugin or style to XenForo, you should have to choose a license from a drop down menu. Either a LGPL-like substance free open source license, which gives the community the right to extend and continue your work (with credit), or the choice of some other kind of license which requires the community to get your written permission before forking your project. Then you give the community enough information to make an informed decision about whether they want to install and commit to your plugin (where they'll be dependent upon you for updates), or choose another plugin which might have a less restrictive license.
I could write a book about all the problems which have come up on vBulletin.org which have been ignored by the administrators, to the detriment of the community (both authors and users).
