Not a lawyer either, but I think there is a more obvious angle to this: publicly traded companies can be audited, publish costs to investors, list their creditors and amount when going bankrupt, ask for tax rebates by declaring their expenses etc. In many circumstances their money flow needs to be transparent.

Not being allowed to expose the buying price of a service would go against many of these procedures/obligations.

We could look at it the same way disclosing one's salary can't be illegal: how do you then apply for a loan for instance ?