One problem is the unreasonable level of control over "second party" use - ie, people viewing it on the web as intended, but in a way that the publisher doesn't like. Whether they're using Linux or screen readers or adblockers or "non-standard" browsers. Even if they're just trying to fix usability problems in the site.

Example: streaming video on the Raspberry Pi; the hardware is quite capable of it but if it requires an x86-only plugin to do so you're out of luck.

Example: "readability mode" in browsers relies on the fact that currently text is not wrapped in DRM and can just be presented how you like it.

Trivial, petty example: currently on Twitter there's no sensible way to view images full size or save them without going via the DOM explorer.