Marketpress & Product theme- When set this overrides the purchase button with a link to this URL

Multisite 3.1, Product Theme, MarketPress

Dashboard>Products>Create New>Product Details>External Link
When set this overrides the purchase button with a link to this URL

Doesn't this give sellers the ability to circumvent Chained Payments owed to the main site if customers can check out on a different site?
If so, how do I prevent this option from appearing?

Thanks.