They are not illegal to own in California. They are only illegal to be on a semi-automatic center fire rifle with a detachable magazine. So if you mag lock your AR with something like this[1] you can put a telescoping stock on it. This is also legal[2] (with a 10rd magazine) since it is not a center fire rifle.
Again a stock is not integral to the operation of most rifles so you can just not put one on. Oh course then it's legally not considered a rifle most of the time.
>Rimfire ammo is limited to smaller calibers since the cartridge walls need to be thin enough to be able to be crushed by the firing pin and ignite the primer.
> The downside?
> Well, the nature of the casing means it’s pretty much limited to small calibers. You have to have some relatively flimsy brass to handle the rimfire set up. As a result, the powder necessary to propel a larger bullet would blow the brass apart.[1]
Correct about the buffer tube. So even if you installed a folding stock on an AR-15, it require some proprietary buffer tube to work like this one.[2]
[1] https://www.pewpewtactical.com/rimfire-vs-centerfire-ammunit...
Telescoping stocks maybe not (my inexperience showing there) but folding stocks definitely can take it under 30 inches then. I assume you're saying a normal caliber ar15 won't be very fireable without a buffer tube. Anyways, apparently folding and telescoping stocks are also illegal in California, which might be the real reason amazon has these rules.