Hi Heting,
Azure DDOS has two SKU. Basic and Standard.
Basic SKU is free of charge and it is enabled to everyone by default to protect Azure Infrastructure.
Standard SKU is chargeable and it doesn't matter you link the SKU with a VNET or not, as soon as you create a plan, you will be billed and that is what happened in your case.
I would recommend you to get in touch with Azure Billing team to see if you can get the credit as you are not aware of the billing of Standard SKU.
Reference: https://azure.microsoft.com/en-us/support/options/
Pricing Details: https://azure.microsoft.com/en-us/pricing/details/ddos-protection/