Your paying customers will start complaining. And switching to other providers. Or making others switch to other providers by publicly complaining.
So, no, there's no obligation other than needing to offer good services to be able to keep your customers.
Let's consider the alternative: you throttle Netflix. This hurts Netflix's business. It will benefit Netflix's competitors. Internet access is a utility, where traffic should be treated equally in order to allow people and businesses to use the internet as a way of providing their services. With equal treatment from ISPs.
If your ISP wants to manage their traffic, they could for example ask their customers money for faster connections, or ask money per amount of data transmitted. Or upgrade their lines.
This post has been posted from a connection through which all traffic is guaranteed to be treated equally by national law. And Netflix works just fine.