The previous decade of Zero motorcycles without 12-volt batteries is evidence they are not actually necessary on an E-bike. I read on this site that at some point recently Zero even added the always-on 12-volt line to the OBDII connector, and that is on an SR without a 12-volt battery. Whether they did that by leaving the main DC-DC converter active at all times or setup a dedicated little buck switch mode power supply, I don't know. I can say that the draw on the main battery pack is irrelevant. On its face, I'd say that choice to use a 12-volt battery is borderline retarded, but I would actually like to see Zero's reasoning behind it because I'm sure they didn't just do it on a whim.
Does Zero still use a DC-DC converter on the SR models? If not, that would explain the use of a 12V battery. But if they still do, such as to recharge the accessory battery, then perhaps they are using a low power output DC-DC that is less expensive than the 500W version like I have in my bike.
I bet that a small 12V battery is a lot less expensive than a big $365 (retail) 500W DC-DC converter. BTW, as I recall the earlier Gen 2 models used a 300W converter and I would expect that even smaller ones are not to hard to source. I also would not be surprised if Zero feels that they need a 12V lithium battery to power the ABS braking system should the bike's power die at speed on the roadway. Stranger things have happened in the past.