Taking a look at this blog post
by cPanel should probably have people using DNSonly quite a bit concerned. Shortly (who knows when, really), they're going to start requiring a valid license for this product.
They claim (of course) that they're not going to start charging, at least yet, but if you look at their blog entry (see above link), you'll see two statements that indicate tht yes, this is eventually going to happen
As we start to look more closely at our customer base, however, we see a significant problem: we lack any real insight into our DNSONLY customers.
Their "reason" for licensing these right there
When asked about "free" licenses:
We need more information before we can make any decisions at all.
So, this licensing information is being distributed to cPanel in order to give them the "information" they need so that they can start charging you for your DNSonly licenses
Thankfully, I ditched cPanel DNSonly quite some time ago, but this is just horrible behavior from this company