being an ex-employee of a northwoods electric utility, this is how it works -- Plattsburg municipal power [and other city owned municipal systems like Burlington's] receive a share of the hydro power produced at Niagara falls. The entire output of that government facility is shared by municipal power utilities from Vermont through New York and into Pennsylvania [and maybe Ohio -- who knows?]. The amount of cheap power so produced is constant and can not be increased.
When Plattsburg, etc. exceed their allotted cheap hydro power, they have to buy or make own power at whatever it costs. Some [Burlington, for example] have other dedicated power sources and/or agreements -- Plattsburg apparently does not. Thus, their excess needs come from whatever source available, even if that's a gas turbine plant located in Quebec which charges free market, for profit rates to American buyers.
Eventually, the Plattsburg "deal" will be remade more like the Burlington deal -- residential use gets first call on the cheap hydro power and commercial use pays more like free market rates ... at which point the cryptocurrency miners may look for another location that isn't yet that smart and still lets these commercial ventures that don't serve the local people in any way take part of the super cheap power for their private profit.