Why is RAID 0 classed as RAID when it's not redundant?

You actually part answered this in your question.

The lowest form of RAID is RAID 1. RAID 0 was added well after RAID was defined (can't find reference to a date for this though)

The 0 in RAID 0 is used to signify that actually it isn't considered redundant. Think of it as more True/False where 0 is False.

RAID is just a name with a meaning that changed over time.

The important part is that the underlying technology and mechanisms are the same for the RAID levels, so you use the same controller (or piece of software, e.g. mdraid) to achieve all RAID levels.

As others have suggested, RAID 0 could be taken as level 0 meaning zero redundancy. It is referred to as RAID even though there is no redundancy for two other reasons:

• It is usually defined and talked about in the same contexts, so the name stuck. The same can't be said for JBOD, but such arrangements don't tend to get described along with RAID levels as they have less in common (RAID 0 at least involves striping as found in the likes of RAID 5).

• While RAID 0 offers no redundancy on its own, it is often used as part of a composite RAID arrangement that does offer redundancy. RAID 10 is the most common, combining 0 for speed with 1 for redundancy. RAID 50 and RAID 60 are less common, but equally valid except where the potential write performance hit (due to 5/6's read-checksum-write pattern) is a concern.