I don't want to speak with 100% certainty, and certainly not for all flavors.
Novell for example will allow you to install SLES (SuSE Linux Enterprise Server)or SLED (SuSE Linux Enterpise, Desktop), but they won't allow even security patches to download / install if you don't buy 'support' for the copy. Of Course OpenSuSE (Novell's 'free offering') does not have this limitation.
Here is a link to their Flyer explaining their Subscription Service
In part from their site:
Your subscription includes:
* Award-winning technical support
* First Look on-demand training
* Maintenance updates, service pack releases, hardware enablement and security and bug fixes Entitlement to the Subscription Management Tool for SUSE Linux Enterprise
* Rights to upgrade to new versions of the SUSE Linux Enterprise family of products
* A generous and predictable lifecycle commitment
* Opportunities to participate in new beta product testing
* Protection under the NovellŪ Technology Assurance Program, including extensive intellectual property (IP) coverage
* Assurance of compatibility with thousands of hardware and software products Flexible, tiered pricing Access to an extensive global network of resellers and distributors
* Access to Novell Customer Center an online interface where, from one location, you can review the status of all your Novell products, subscriptions and services, and obtain critical Linux updates and support
Frankly, I think it is a bit of a stretch for them to say they follow the GPL, since they release the updates as well, but only to paying customers. But that is how they attempt to (currently do) get around it. You can download and install it if you want (for free), but if you want updates, security . . . then you have to buy the subscription.
Bottom line for me at least I would rather run SLES, with OES (Open Enterprise Server) and Groupwise, and avoid the Closed source, overpriced world of Microsoft. Though I run a lot of truly free systems too.