Wouldn't that be quite challenging in terms of engineering? Given these people have been chasing AGI it would be a considerable distraction to pivot into hacking into the guts of the output to dynamically push particular product. Furthermore it would degrade their product. Furthermore you could likely keep prodding the LLM to then diss the product being advertised, especially given many products advertised are not necessarily the best on the market (which is why the money is spent on marketing instead of R&D or process).
Even if you manage to successfully bodge the output, it creates the desire for traffic to migrate to less corrupted LLMs.