On the checkout page requiring a WordPress.com account, I see this message, with a link that explains the need for the WordPress.com account:
WooCommerce.com now uses WordPress.com Accounts. Learn more about the benefits
Note that Woocommerce (as well as the Woocommerce Subscription plugin) is owned and developed by Automattic, Inc. Automattic also owns WordPress.com.
I believe the WordPress.com account is only for unified account management across their various products (rather than have different logins for each product: Woocommerce, JetPack, WordPress.com, Akismet, VaultPress, etc)… and you should be able to use the product you purchase on your self-hosted WordPress site.
You don’t have to take my word though: I encourage you to contact Wooocommerce.com themselves for an official response: https://woocommerce.com/contact-us/
Good luck!