Under the 1998 Digital Millennium Copyright Act, Tumblr, YouTube, Reddit, WordPress, and Facebook aren't responsible for the copyright infringement of each of their millions of users, so long as they take down specific posts, videos, or images when notified by copyright holders. But copyright holders thought that wasn't good enough.
The FCC banned throttling for good reason, namely that Internet service providers should not bias their networks toward some applications or classes of applications. Biasing the network interferes with user choice, innovation, decisions of application makers, and the competitive marketplace.