It's not being throttled. There is a peering dispute between netflix's CDNs and the ISPs. Netflix's CDN's don't want to pay for delivery to the ISP. Normally CDNs peer with each other for free if the balance in each direction is equal. The problem is that any CDN that delivers netflix is hugely unequal since it's a huge one way stream.

The ISPs could just eat the cost, and let it be delivered for free. Or Netflix's CDNs could just pay the peering fee. Or Netflix could use different CDNs.

The reason the VPN works is because it sends the data over a different CDN that isn't saturated because of the dispute.

At least that is how I understand it, but I'm not an expert (or even a novice).