https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/lock-resources Applying ReadOnly can lead to unexpected results because some operations that don't seem to modify the resource actually require actions that are blocked by the lock. The ReadOnly lock can be applied to the resource or to the resource group containing the resource. Some common examples of the operations that are blocked by a ReadOnly lock are:
A ReadOnly lock on a storage account prevents all users from listing the keys. The list keys operation is handled through a POST request because the returned keys are available for write operations.