From a technical standpoint it’s far easier to be net neutral than throttle traffic which takes processing power, energy, and programmer time to do so. This is entirely about the ISPs making yet more money.
There's a lot of revenue in charging both sides for connectivity, so it doesn't matter which is "easier", only what is more profitable.
What do you need to have in place if someone alleges you are not neutral? Your accuser might present well collected evidence that Hulu performance to your customers is worse than Netflix's performance. You will then claim "but we didn't do it, it is just the luck of the draw on how we choose to buy interconnect bandwidth". Is that good enough to protect you? How can you prove you didn't choose the interconnects to screw Hulu?
So there is a burden. If there is no requirement, even if you never take money to slow down someone's competitor, you are relieved of this burden.