[davidxc]

Look up tax incidence and elasticity of supply / demand curves. Yes, it could definitely be reasonable to assume, from a modeling perspective, that companies currently charge the optimal price for maximizing profit. However, once you impose extra transaction costs on companies (like a tax), the equilibrium price will then change (previous equilibrium is no longer the equilibrium because external state has changed).

By your argument, taxing a transaction in a market would not ever raise the price charged to the demand side. It's pretty easy to see by reading a chapter about tax incidence (which is different from where the tax is legally placed) and elasticities of supply/demand curves that this is definitely false (not just in theory, but also in practice)