It appears the registration is only required for "cloud" features managed through Netgear's subscription service. Here's what's possible without the cloud service: https://kb.netgear.com/000061174/What-features-of-my-NETGEAR... And here are the features of the cloud service: https://kb.netgear.com/000044342/What-can-I-do-with-Insight
From what I'm reading, it seems like you're required to register some devices or their functionality will be reduced, and then if the device can also be managed through the cloud interface (regardless of whether you actually want to be managing it that way), it will count against the free device limit (as long as you register it under the same account).
However, it also looks like these products were advertised as such from the start. I previously thought that this was forced upon the users through a firmware update.
1. https://kb.netgear.com/000048452/What-devices-can-I-discover...
2. https://kb.netgear.com/000053256/What-Insight-subscription-p...
3. https://kb.netgear.com/000053255/What-countries-and-currenci...