The viewpoint is this. The ISP is responsible for relaying internet traffic to their customers - they have different models for this, by "speed", by amount of data, etc and this is working reasonably well.
Then new suppliers of content arrive, heavy 4k streaming - content which is far more susceptible to "errors" in transmission than in ye olde days. Streaming of near lossless, high resolution video requires both low latency, high average throughput and high instantaneous throughput. This puts a different kind of stress on the system, people streaming 4k video consumes more resources than what the same amount of data would require in a different setting. The main problem is that the supplier of content (i.e Netflix) has no incentive to avoid these issues, to design their content optimally. Quite the opposite. This can put a lot of stress on specific parts of the network - and again the supplier and customer both can just blame the ISP. The ISP will take steps, certainly improve transmission, maybe work out some caching or storing at different sites, but the cost will be evenly distributed on all customers. Charging extra for "unthrottling" allows for the customers that consume the service to pay for it.
Now, this is a minefield - there are huge pitfalls everywhere and throttling can certainly be abused in a number of ways, especially when we know that the service provider market has a significant barrier to entry and has had problems with near monopoly situations in the past. However, the argument against net neutrality is simple - why should the general customer pay for a service (from the ISP) that only benefits customers of certain content suppliers?
If Netflix were to offer airline passengers the option to watch Netflix during their flight, pass the cost of satellite usage on to United Airlines, making the tickets more expensive for everyone - it is to me obvious that we should allow United Airlines to charge Netflix for this inconvenience.
In closing, as I started understanding this from the other perspective my view of Net Neutrality changed. I still believe in a free, neutral and open internet - but we need to make the ISP service providers work in a reasonable environment as well or else we dig the grave even deeper. Not allowing for price differentiation will make the barrier to entry even higher. The market could use some regulation, and it needs to be good regulation in order to serve both customers, service providers and content providers.