The goal isn't to have an ad->purchase, the goal is to make sure the purchase is more likely in the long term.
I think if you had this incredible technology that could manipulate language to nudge readers in the softest possible way toward thinking a little bit more about buying some product, so that in aggregate you'd increase sales in a measurable way that nobody would ever notice, it would just quickly just devolve into companies demanding the phrase "BUY MORE REYNOLDS GARBAGE BAGS!!!!!!!!" at least 7 times.
This is the machine that magicians program for: https://www.youtube.com/watch?v=wo_e0EvEZn8
I was going to write a rebuttal to this, about how more subtle forms of advertising are likely not very effective, and then I remembered subliminal advertising.
It's largely been banned (I think), but probably only because it's relatively easy to define and very easy to identify. In the case of LLMs, defining what they shouldn't be allowed to do "subliminally" will be a lot harder, and identifying it could be all but impossible without inside knowledge.
How effective is it? We don't know, but there is nothing of potential value to lose so nobody really cared. Just ban it and move on.
Oh come on.
Genuinely.
Come on.
Look at every single tech innovation of the last 20 years and say that again.
