Why is vpcFlowLogsNotEnabled determined to be a violation?

[hiromasatakakuma]

• terrascan version: 1.1.0
• Operating System: macOS

Description

When I define a aws_vpc and aws_flow_log, vpcFlowLogsNotEnabled gives a level medium violation.

What I Did

$ cat vpc.tf
resource "aws_vpc" "example" {
   cidr_block = "xxx.xx.xxx.xxx/xx"
}

resource "aws_flow_log" "example" {
  log_destination      = data.aws_s3_bucket.example.arn
  log_destination_type = "s3"
  traffic_type         = "ALL"
  vpc_id               = aws_vpc.example.id
}

$ terrascan scan -t aws -f vpc.tf
results: results:
   violations:
   --rule_name: vpcFlowLogsNotEnabled
     description: Ensure VPC flow logging is enabled in all VPCs
     rule_id: AWS.VPC.Logging.Medium.0470
     severity: MEDIUM
     category: Logging
     resource_name: example
     resource_type: aws_vpc
     file: vpc.tf
     line: 1
   count:
     low: 0
     medium: 1
     high: 0
     total: 1