"In reality the license just puts extra restrictions on distribution of the software and how derivative works can be licensed."
Actually, copyright is now implicit. There is NO right to distribute software and NO right to create derivative works. Copyright gives those rights exclusively to the copyright holder.
The GPL makes it legally possible to distribute and create derivative works. Without that license, you have no such right, and can be sued (for damages) by the copyright holder. Proving damages can be difficult.
In any case, the license is the only thing that allows that behavior. Without it, there is no right to do it, and thus restrictions are moot.
There is a fun thought experiment where one takes a copyright license and rewrite it in terms of only positive permissions. Since copyright law by default forbids everything, every aspect of a copyright license is a form of granted permission.
The GPL would then be a number of permissions similar to:
You are hereby given permission to give other people a all-permissions-granted patent license of all patents that you own, and along side that license you are given permission to include my copyrighted source code, and optionally in addition to that source code you may then add a compiled version along side that source code.
No restrictions and depending on how one want to interpret "in addition/along side", not even a condition.
Actually, copyright is now implicit. There is NO right to distribute software and NO right to create derivative works. Copyright gives those rights exclusively to the copyright holder.
The GPL makes it legally possible to distribute and create derivative works. Without that license, you have no such right, and can be sued (for damages) by the copyright holder. Proving damages can be difficult.
In any case, the license is the only thing that allows that behavior. Without it, there is no right to do it, and thus restrictions are moot.