TJ Hooker :
Why is this even an option, let alone the default?
Facebook's problem is convincing advertisers that the information they gather truly reflects the purchasing habits of its users. Venmo is actually showing transactions, places and times you have actually purchased something. Link that with a Facebook ID, and you have an incredible insight into a potential customer.
Imagine showing an ad for a product on a persons Facebook page, and seeing that 30 minutes later they make a related purchase. That's feedback few advertisers get for real-world purchases.
Oh, and anyone thinking Venmo's features are for the users is probably not fully understanding how today's internet works. If the service is free, someone is making money off the information you give them.