I don't think that's true anymore, see https://www.cloudflare.com/ssl ("Full SSL" and "Full SSL (strict)" options)
Why would they do that? I never used this feature of CloudFlare, but it would be only logical to require you to upload the cert so they can verify against it in the future.
Use this to serve e.g. an S3 site on your own domain using SSL: S3 -> CloudFlare (with CloudFront) uses Amazon's certificates for their hostnames. Cloudflare -> internet uses your website's hostname and certificates.
Total money spent on SSL: $0.
https://blog.cloudflare.com/universal-ssl-encryption-all-the...