Indeed, WooCommerce works with many payment gateways. However, upon installation of WooCommerce, if you go to Settings > Checkout … Simplify Commerce is definitely the default gateway provided.. for free. As in it is shipped and installed with WooCommerce. In order to use a different gateway, one must pay for the upgraded gateway, and install the plugin by uploading the zip file.
It’s my assumption that most who are say, using all the defaults for their calendar, are likely just going to set up with the pre-installed payment gateway, which up until just a few weeks ago didn’t disallow ticket sales. Which means everyone in that situation is going to get their CC processor silently shut off and they won’t know until the get a letter in the mail.