In most areas of the law, anything that is not prohibited is allowed.
In regulated industries, such as the sale of electricity derived from the electrical grid, the reverse pattern is the norm. Everything that is not expressly authorized and licensed is prohibited.
In particular, utility regulators always regulate prices and that means that the regulator needs to publish the allowed price that it decrees following administrative hearings, and that means it needs a simple pricing structure.
Kilowatt hours pricing was developed to keep the market for electricity simple enough for regulators and consumers to not be outwitted by complex schemes by regulated utility companies, and once it became customary and prescribed, pretty much every utility regulator followed suit, although some utilities now allow peak and non-peak price kilowatt hour price variations.
Aa EV-charger business is a participant in the regulated industry of providing electricity and thus, is a utility, even though it is not your classic image of one (secondary market sales of electricity to consumers is new, although secondary market sales of electricity at the wholesale level have actually been around for a century or so). Therefore, they are stuck in the same pricing structure that utility regulators have in place until legislation or innovative new regulations change the status quo.
Also, keep in mind that for a charger of a given voltage and amperage, per minute charges are exactly proportional to per kilowatt hour charges, based upon a formula you can derive from basic electrical power formulas. So, it is easy to see how the two concepts would become muddled.