According to the document the FCC released trying to justify this stuff, ISPs can't always figure out how do that, so they need paid prioritization: "Without paid prioritization, ISPs must recover these costs solely from end users, but ISPs cannot always set prices targeted at the relevant end user".

See paragraph 252, which starts on page 142, of this document: http://transition.fcc.gov/Daily_Releases/Daily_Business/2017...

Here's the full paragraph in case the FCC site gets slow (their file hosting wasn't designed for a world where people are actually interested in FCC documents):

FCC> 252. Efficiency. We find that a ban on paid prioritization is also likely to reduce economic efficiency, also likely harming consumer welfare. This finding is supported by the economic literature on two-sided markets such as this one, and the record. If an ISP faces competitive forces, a prohibition against two-sided pricing (i.e., a zero-price rule), while benefiting edge providers, typically would harm both subscribers and ISPs. Moreover, the level of harm to subscribers and ISPs generally would exceed the gain obtained by the edge providers and, thus, would lead to a reduction in total economic welfare. The reasons for this are straightforward. Some edge services and their associated end users use more data or require lower latency; this may be the case, for example, with high-bandwidth applications such as Netflix, which in the first half of 2016 generated more than a third of all North American Internet traffic. Without paid prioritization, ISPs must recover these costs solely from end users, but ISPs cannot always set prices targeted at the relevant end users. The resulting prices create inefficiencies. Consumers who do not cause these costs must pay for them, and end users who do cause these costs to some degree free-ride, inefficiently distorting usage of both groups. When paid prioritization signals to edge providers the costs their content or applications cause, edge providers can undertake actions that would improve the efficiency of the two-sided market. For example, they could invest in compression technologies if those come at a lower cost than paid prioritization, enhancing efficiency, or, if they have a pricing relationship with their end users, they could directly charge the end user for priority, leading those end users to adjust their usage if the user’s value does not exceed the service’s cost, again enhancing economic efficiency. And to the extent an ISP has market power, antitrust law would only allow such ISPs to engage in pro-competitive paid prioritization practices.