Can you educate me? Why is this considered a bass?
I'll play. Having owned several 6-string basses since 1990(and with another on the way) I should be able to answer your question. It is a short-scale(30") bass with 2 extra strings above the normal 4. String manufacturer's set guages vary but they are usually lighter guage than standard short-scale strings. I usually use a .95 low E string on my 6s. Due to the close string spacing they are usually, but certainly not always, played with a pick.
The 6-string bass was introduced in the 50s by Danelectro. Leo Fender saw the Dano and came up with the Fender VI. Several other companies produced 6-string basses including Hagstrom, Rickenbacker, Epiphone, and Gibson.
They were popular in the Country music at the time, used to double the stand-up bass part on recordings, to accentuate the bass part on car radios.
Just off the top of my head here's a few folks known to have used a 6-string bass:
The Hollies
The Who
The Byrds
Soft Machine
The Beatles
Cream
The Band(still known as The Hawks at that time)
Plenty more but that's all I can think of right now.