The Choose a Venue step shows up on the catering ordering page whenever the Venues module is on and venues are allowed for catering, regardless of the Mandatory/Optional/Not Allowed setting. What that setting (Catering: Ordering On Venues) actually changes:

• Mandatory: once you have catering items in your cart, checkout will pop up a "Select a Venue" prompt for that date until you pick one.
• Optional (the default): after answering Yes to "is this at a venue," you're also asked a follow-up "Do you want to Add Catering?" Yes/No question before the meal picker appears.
• Not Allowed: you can't add catering meals to a date where a venue is selected — the system blocks it with an error ("Catering Is Not Allowed On Venues").
• Saying Yes to a venue always reveals a guest-count field and the venue browser.