Within the last century, electronic communications have increasingly become the vehicle of democratic discourse. Because radio and television broadcasting are expensive with limited frequencies available, the wealthy have dominated broadcasting. The Web places into the common man's hands the capability of global electronic broadcasting.
To me the issue is that DRM (and its twn, the abdication of net neutrality) will allow content providers (=big companies, usually) to control content in ways that conflict with: 1: how their customers want to use that content, and 2: more importantly, the open culture of the internet that allows people to share content liberally, which includes political speech. Such content control will make it more difficult to use the internet as a platform for expressing political opinions and for accessing such content. This is serious because the internet is the first technology that has allowed this content to be disseminated rapidly and relatively unobstructed by national borders and the first technology to make it easy for anyone to do so; giving large content providers extensive control over how content is created and accessed (and over who can do it, on the net neutrality side of the coin) will make it more difficult for ordinary users to engage in political discourse on the internet (because these companies will be in control of the technologies used to access said content) and will turn the internet into another medium with high entry barriers, which will restrict it to those who have the money to provide the content. Just like, say, mainstream newsmedia. Of course, in that case there are natural limitations (limited bandwidth); in this case, it would be an artificial constraint that completely ignores and defeats the internet's ability to facilitate and enable communication and content creation.