The way these things tend to work is "It's GPL, but if you redistribute it publicly we terminate your subscription with no refund." (I think RHEL binaries work the same way, for instance.) The reason for companies to pay is to get reliable updates for new versions, so that they can avoid hiring a bunch of people in-house to build / forward-port things. So usually this incentive works.

Now that grsec is completely unavailable without a paid subscription, either source or binary, it'd be interesting to see if the incentive holds up, or whether someone wants to burn a subscription to get a version out to the public. I imagine that they're cautious about who they allow to sign up as a customer.