Because net neutrality applies only to how the bits flow over the physical wiring (and radio signals) of the last mile. NOT the backbone. That's what it's about. Backbone operators can and do have peering contracts and give priority to some companies and not others.
ISPs can block all P2P traffic (bits flowing between residential IPs) and removal of Net Neutrality regulations will embolden them to do so, but it's also about the trend toward ISPs being giving the green light to do whatever they want, with eventually no regulation whatsoever.
