The Times Union piece you cite does not say that the union supported lowering standardized test scores required for graduation. In fact, the article quotes the union funded Alliance for Quality Education as opposing the change.
Charter schools create duplicate systems with duplicate costs, reduce funds available for traditional school students in sending schools, and are a waste of taxpayer resources. When students leave traditional public schools for charter schools, the traditional public school is unable to reduce costs in proportion with the loss in per-pupil revenues. Students in public schools are left with fewer resources.