Almost all the non-major online ebook viewers are like that.

Some of them would obfuscate the image resources, but that's about it.

Big players typically have something "better". Bookwalker (a Japanese ebook vendor) has the most complicated viewers in my experience, and it always convert all the text to image on server side before serving. (And it obfuscates the images on server side too, so you have to reverse this process yourself if you want to download them).