whenever you want you can always throw the credit card merchant agreement at any of those vendors just because you feel mildly inconvenienced.
it is against the merchant agreement to have any purchase minimum and vendors have absolutely no leverage on this reality (as this is per payment network, this can change per network at any time, but my understanding is that merchants can't have card minimums - this also applies to bars where much higher card purchase minimums are also commonplace)
of course, the sympathetic approach is to realize that its more profitable for the credit card network to have no mininum. but as you don't really experience this concept of card minimums outside of NYC, there's no reason to follow along with those merchant's rationale just because they told you why and you feel sowwy for them.
when I think about every thing that threatens low margin businesses, I just notice that a different one will take their place and a few owners will have to do something else to exchange time for food and shelter just like everyone else, so no need to keep them on life support