Whether not taxing a particular thing is right or wrong is a totally separate issue.
Good enough for me.
Edit: WTO definition of subsidies includes "government revenue that is otherwise due, foregone or not collected (e.g. fiscal incentives such as tax credits)". Pdf: http://www.wto.org/english/docs_e/legal_e/24-scm.pdf
Do you have a reference for that claim? Other sources explicitly count tax breaks as a type of subsidy, for instance
"In countries with well-developed tax systems, subsidies provided by reducing companies' tax burdens are commonplace. Examples include tax exemptions (when a tax is not paid), tax credits (which reduce a tax otherwise due), tax deferrals (which delay the payment of a tax) and a host of other instruments. In common language these preferential tax treatments are called tax breaks or tax concessions; public-finance economists refer to them as tax expenditures."
https://web.archive.org/web/20120905082615/http://www.iisd.o...