Haha yes I suppose this is confusing, I'm not talking about metered taxis but rather minibus taxis which operate more like public transport. They only drive certain routes and you just get on and off wherever along the way. At the moment there are a number of reasons why expecting them to have card machines would be strange but in terms of the future it's certainly possible that this might change.
As for the meter taxis, here there are two options: you can either use the fancier "brand name" guys which are starting to have card readers more often, or the pirate style guys who drive busted 1990 Camry's and take the taxi sign off the roof when the cops are nearby. Needless to say they don't take plastic.
As for POS I was under the impression that certain account/card packages do carry charges for the user. Otherwise I don't know why they would list it as a charge in the brochure? Like you say most people (myself included) don't worry about the cost to the vendor so ja...
Minimum transactions are just a thing here then I suppose, it might again have to do with preventing people from making large numbers of card transactions and racking up fees for the business - perhaps the fees are relatively higher?