Because they are an expert in their field and the client, presumably, isn't? I can't imagine another field—hairdressing, construction, financial advice—where the client would reject the paid expert's viewpoint so readily and firmly.
The client here is just requesting specific content on their website, similar to someone requesting a granite countertop in their kitchen; that seems fine, even if its not particularly classy or aesthetically pleasing to the contractor.
To be clear, I would personally have a similar view to the author here. I'm just surprised that they think their opinion on the strategy side matters so much to their client!