It's a very good point. The GPL is a license you can choose if you wish to do something that would otherwise breach copyright law. The loading and execution of copyrighted code wouldn't normally require a copyright license, so you can choose to disregard the GPL or any other license that will grant you extra powers.
For example both AGPL and GPL3 say
You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a consequence of using peer-to-peer transmission to receive a copy likewise does not require acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.
GPL2 has a similar clause
To me, I get the feeling the drafters of the license believe that modifiying code, even privately, and not distributing it any further than your own laptop, requires acceptance of the license, thus modifying AGPL code would require acceptance of that license.
I'm not convinced that's correct, but maybe courts see it that way. After all I suspect if I modify a copy of close source software (to say bypass some security lock) that I obtain in accordance with copyright law, not making any copies (for the purpose of copyright law) of it, I would be in breach of copyright.
If that's the case, then if you don't modify an AGPL software and just run it, you don't need to agree to the license, but if you do modify it you are in breach of copyright law unless you agree to the license.