The best reason I can think of for NOT sliding on a 12-string is that the damn things are so hard to retune, and slide guitar usually involves use of at least two open tunings, Vastapol (down to open-D or up to open-E) and Spanish (G or, rarely, A).

I once posted a message observing that 12-string guitars are not just twice as hard to tune as sixes, but four times as hard. I was corrected by a 12-string owner, who notified us all that twelves are 144 times as hard to tune as sixes.

There is such a thing as a 12-string resonator -- National makes 'em, if no one else, and shows them on their website. Of course, resonators can be played without a slide in standard tuning, but I think most buyers get them because of the sound they make with the slide.

Oh yeah, one other thing. Some people "convert" twelve string guitars to sixes simply by installing only six strings. Could that be what you're seeing in that picture? The twelve tuning machines on the headstock are highly visible, of course, but you may not even be able to see whether all 12 strings are there or not