"Buts not the same as any other registry... because its not actually a registry. Its a fee based savings plan."
As DavidH mentioned, I'm defining its similarity to other registries in an etiquette sense, because whether it's a sound financial idea is outside the scope of etiquette. If you find it distasteful, then there's nothing forcing you to use it, just as would be the case if someone registered for a gift that you found distasteful.
"To me, the biggest difference is that people on this car site write blurbs to justify their request, and they come across as incredibly entitled. For example, "I've been a good student so I deserve a car". You won't see this on a wedding registry, or even an Amazon wish list."
There's nothing that requires an entitled blurb to set up this registry (one's blurb could easily just say "This is X's registry"), so it's the blurb that would be rude, not the registry.
"Also, a wedding registry is event specific, and is meant as a guideline or even a convenience to guests. Weddings are pretty much gift giving events, and the registry helps with ideas. That seems very different from telling people you want a car and you want help paying for it."
That depends how one uses it. If it was used much like a wedding registry, as a guidline for a specific event, then it would be identical to a gift registry.
"Finally, there are specific and long standing rules on how wedding registry info gets disseminated. It can't go on an invite, but it can be related via parents, a bridal shower, and the bride herself if asked about it."
This registry can be disseminated in the same way as a gift registry.
"With something like this, there's no way for its existence to be made available unless the person setting up starts that process. How do you start the process without begging for cash?"
That's no different from a gift registry. How would anyone know where you registered if you didn't tell them? As long as you tell them only when it's appropriate, again, it's no different from a gift registry.
"And maybe it's an uncharitable assumption, but I'd think once people start telling about it, it's just a short leap to posting it on FB."
It's interesting at least, and again, one could do exactly the same thing with a gift registry, and it would be rude to do it. But there's no requirement that this registry be used any differently than any other registry, so it's not de facto rude to use it at all. It may not be smart, but it's not rude.