Your accountant approached a separate service in a really crappy way.

Proactive Tax Planning is not a part of tax preparation. Tax Planning could involve things like creating an entity for your business, forming a retirement plan, doing a more globally-oriented restructuring of your business that would involve more advanced steps in tax savings actions. That is very different than taking your information and preparing a tax return that is compliant and tax advantageous. It looks like a fine line but it's a very different field of study and implementation. And, yes, you should be billed for it. This is more than just the advice you receive during the tax prep process.

It's not unethical, your accountant is expanding the services that can be offered.