I went to look at Visa rules and I think you are right. Here's one relevant paragraph:
> 5.4.2.3
> Minimum Transaction Amount – US Region and US Territories
> In the US Region or a US Territory, a Merchant must not establish a minimum Transaction amount as a
condition for honoring a Visa Card, except for a Transaction conducted with a Visa credit Card issued
in the US Region or a US Territory.
The minimum Transaction amount must not be greater than USD 10 and must not be discriminatory
between Issuers or between Visa and another payment network.
So, in the US, it doesn't look like there are any rules. Outside of the US, it looks like a minimum is fine as long as it's not more than $10
