The individual who is signing the person up is verifying that person. This is creating a Proof of Human concept. If I am going to use my RC, odds are that it will be with someone who I know exists. Applications are going to be under the same constraint since it is in their best interest to use their RC wisely.
There is a valid argument behind this. I remember when i joined gmail in the early stages you needed to be invited in by an existing member. They were only allowed a total number of invites so obviously they weren't going to waste them unless they wanted to help this person. The same for me joining since gmail was a better standard of email account than anything that had been there beforehand so obviously it was a status and a benefit to join the service.