The Green Party has many environmental concerns about supermarkets including their impact on local high streets and the high carbon footprint of their imports. In terms of social justice their low wages, zero hours contracts and extensive use of free labour via workfare are a major issue.
This BBC report brings home how this exploitation boots supermarket profits. Back in 2010 the Green Party's evidence to 21st Century Welfare commented:
Large employers (e.g. supermarkets) benefit from a wage subsidy via tax credits which they do not need. The effect of this subsidy should be reserved for small firms, by requiring large companies to contribute to a special social investment fund with contributions inversely dependent on the margin of their wage rates above the minimum wage, so that those paying least (and thus depending more on the existence of tax credits/benefits to make recruitment viable) would pay more. This fund would be used for social housing investment and to create jobs in voluntary organisations. Large companies would be encouraged to do these things anyway and to benefit from any associated publicity from their sponsorship.