If Event is setup w/ PayPal Pro as the payment processor, then the Event registration forms should NOT validate against the Billing Info fields when PayPal Express button is clicked. This is working properly on Contribution page - but not on event registration pages.
To recreate - go this page on demo, enter email address and click PayPal express button. You'll see form field validation errors on the billing fields:
If you try the same on this page, it works properly (forwards to PayPal site):