Encoding fair use via DRM shuts out other activities that otherwise fall under fair use. If fair use is to mean anything, its application must not be dictated by the copyright holders, but by Congress (with oversight by judiciary). This is a simple application of the principle of not setting the wolf to work guarding the henhouse. Every five-year old knows this.
Enshrining a particular set of rights in code shuts out other rights that should also apply. It also keeps the rights shut out when Congress eventually sees the light, and effectively limits Congress's ability to set copyright law. DRM effectively neuters Congress.