So you got me curious enough to try to call up my three local ice cream places.
Spot 1: No phone number on either Google Maps or their website.
Spot 2: This was the spot I had in mind with my previous comment - indeed, they do not sell by any quantity larger than 473ml pints (I typo'd previous comment - 16oz, not 19oz pints.)
Spot 3: Happy to report that they will sell by the 3 gal bucket (or half bucket) for $150/$75, respectively.
re: Spot #2 and your "what's wrong with" question - this is a trendy place that opened about a decade ago, and I expect they're just not interested in taking lower margins or complicating their offerings - it's simply "take it or leave it" on their pints.