To suggest these are similar (which, to be fair, you are not explicitly claiming) seems to be to not understand the nature of what the banks are doing.

In investment banks, as a client you pay the research department for objective research into a security. By preparing research that is intentionally untrue, in order to benefit a deal team, there is a violation of duty from the seller of goods (the equities analyst) to the paying client (money manager). In today's world, that violation of duty is not only moral but legal as well.

The equivalent here would be you, as an angel investor, hiring a YC representative as an investment advisor on what companies to invest in, and the YC rep pumping whichever company in the YC program they wanted to get money for, without even disclosing conflicts of interest.

That same YC representative giving publicity to the general public ahead of demo-day has no financial conflict of interest at all. I have a hard time seeing what moral (much less legal) duty they have to not publicize their portfolio companies.

There's an argument about YC reps giving favorable publicity to some but not other companies being unfair signaling, and disadvantaging the companies who are given less hype time. I don't know if I agree with that argument, but I think it's a valid point to discuss. Furthermore, YC partners are locked out from investing in YC companies for a period of time, for a similar reason, so the argument gets stronger.