Buildings can only convert to one culture, so we made a judgement call and made it Steppe Nomadism. The primary gameplay intention is that it's rarely the culture you want (no conversion for Sauromatae because that would be helpful to them, no conversion for the other steppe factions or Numidia until after their reforms), so it creates unrest. That's the trade-off - a "foreign" culture in return for more units.
North Africa in this time was actually steppe, not desert, which is why Numidia's native culture is Steppe Nomadism, not Arid. The Sahara hadn't advanced as far north as it has now, too. So that leaves only the Arabian Enclave that's not a great fit.