That only applies to cross listed symbols (e.g. Blackberry), currencies, etc. The fact that one can shift trading in cross listed symbols to other venues doesn

That only applies to cross listed symbols (e.g. Blackberry), currencies, etc. The fact that one can shift trading in cross listed symbols to other venues doesn't mean that supply&demand also doesn't reduce market making (thereby increasing costs). In fact, Bernie Sanders explicitly hopes the tax will do exactly that.