– Why does a customer need to create an account at checkout?
Subscriptions persist beyond a transaction and therefore must be associated with a user account.
– Why only one subscription per transaction?
The current version of WooCommerce Subscriptions supports only one subscription being purchased at the same time (though multiple subscriptions can be purchased by the same customer in separate transactions). This is because the payment gateways Subscriptions was originally written to work with, specifically PayPal Standard, do not support purchasing more than one subscription at a time.
However, as Subscriptions has evolved and can now be used with many more advanced payment gateways, the ability to purchase multiple subscriptions in the same transaction is planned for version 2.0. The estimated release date and other progress on this feature is documented in the Subscriptions Feature Roadmap.
For now, to enforce one subscription per transaction, when a customer clicks Sign Up Now, Subscriptions removes any subscription already in the cart and sends the soon-to-be subscriber directly to checkout.