The way I think of it is that with permissive licences the freedoms can be removed from the code, but with copyleft the freedoms can't be removed from the code. By freedoms I mean the freedom to study the source code, to run it for whatever purpose you want, to make modifications and to share the code with other people.
> people have the right to ask you for the source code.
People you distributed binaries to have a right to the source code via the same method that the binaries are provided in. Or at least that was my understanding.
If it's a non-commercial affair, you can give them a "written offer valid for at least 3 years" and then you don't have to actually supply it unless someone asks.
In practice, this isn't a very good option to take. Much better to provide it on a "network server" as they were called at the time the license was written.
"b) Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by a written offer, valid for at least three years and valid for as long as you offer spare parts or customer support for that product model, to give anyone who possesses the object code either (1) a copy of the Corresponding Source for all the software in the product that is covered by this License, on a durable physical medium customarily used for software interchange, for a price no more than your reasonable cost of physically performing this conveying of source, or (2) access to copy the Corresponding Source from a network server at no charge."