From what I've read, (I haven't followed it too closely as I'm not from the US) the issue is that repealing NN enables an ISP to throttle Netflix for users who don't pay more for (for example) a 'Media' plan WITHOUT even needing to make a deal with Netflix. Monopolies then become a problem because certain areas of the US can only get connection with one ISP, so there is no room for them to choose a better option if their only available ISP starts charging through the roof for access to specific websites.
I could be completely wrong, but this is what I took away from most of the pro-NN arguments.