Announcing it ahead of time doesn't make a difference because the market still doesn't know what the material information is or even whether it exists. It's like management saying "I know something you don't knowww ;)" and leaving the market to sort out whether or not that's a meaningful and reliable piece of information.

My point is that there often isn't any material nonpublic information underlying the buyback so it only serves as a way to juice the stock price in the short-term by manipulating the psychology of the market. It's also worth noting that these transactions were illegal until Rule 10b-18 specifically exempted them from market manipulation and deception rules.