People in line almost never know what they want. So they all have to wait until sampling several flavors before deciding. It's also an opportunity to upsell. So it's not in their best interest to allow ordering ahead of time. If you know what you want for a pint, you can already skip the line by going straight to the fridge and then to the cashier.
If you just want to buy a pint of their ice cream you can walk in, get it out of their freezer, and go to the payment area. They use iPads for payment but not orders. They want you to taste and then buy.
I have thought the same about Voodoo Donuts. It's my guess that they only want to hire so many people and they can only help so many people at a time. My theory is if you have to wait in line it must be worth waiting in line for.
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