Posting your story online takes the risk that people will copy the text of the story and post it elsewhere, but it is very rarely done except by automated bots. Those bots can get Wattpad and Webnovels story in literal seconds, too, so the lack of copying on those sites does absolutely nothing to prevent the actual problem plaguing web fiction. In fact, I myself at this very moment on the program Calibre could download my own stories from Wattpad into ebook form to put on my Kindle with almost no effort.
If you are actually worried about control over your own content, you should worry less about how easy/hard it is to steal it, and more about how to make your story even more visible on these sites because that's all that really matters for free web fiction. Also, don't use webnovel.com, it's got some really super shady stuff going on.