It's not all transactions, just social ones. People share their personal information on social media all the time, particularly information that relates to a social interaction. Since money is often a component of social interactions, a public record of social transactions essentially adds no information to what people have already voluntarily posted on other social forums. I use Venmo all the time, and since the dollar amounts aren't shown (only the participants and a short memo) I generally feel comfortable with public transactions. More sensitive interactions I make sure to set private.
A friend once paid me back for a "sensitive interaction" and put a snowflake emoji in the comment. I don't know if it was public. It was tacky and thereafter I only took cash for such transactions.
When I scroll through my Venmo feed, all I see is "Mike paid Jen for pizza".