Standing - why Stallman didn't do an explicit contract

> 5. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.

There's room for argument that this is a little too implicit and "automatic" to really qualify as a proper contract, but the flip-side is that an explicit "clickwrap" contract would make dual-licensing more complicated, would require UI support, would make batch installation tedious, etc. Arguably, the GPL's wording strikes a reasonable balance here.