Yeah, the laws against subliminal advertising were written in a rather knee jerk reaction to the creepiness of the entire concept instead of as a result of careful study and analysis.
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.
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.