According to TikTok's community guidelines, it is not allowed to share, post or upload content that infringes copyright, intellectual property rights or trademark rights. And posting someone else's content as your own is directly a copyright issue. This is exactly why TikTok puts a watermark on all videos that are going to be downloaded. In conclusion, the visibility of the watermark demonstrates that you are aware that the video is from a third party and that you must give credit to the original owner.