The registration form is only shown in Event Calendar Pro if you have Authorize.Net enabled because we need to capture the customer information before submitting the card information when using Authorize.net, otherwise the customer information form is not even shown in Event Calendar Pro.
If you have both Authorize.NET and PayPal enabled the form is shown but users do not need to fill it out in order to pay with PayPal, they can just click the PayPal button and go directly to PayPal, it is not required that they fill in the form before doing that. Same with google checkout.
Don't you have a PayPal account? I assume you do since you must have one to also use PayPal on your own site, so I believe when you got to PayPal they knew who you were based on an existing cookie so they were able to pre-fill the form.
Hope that helps,