IANAL, but I don't think you're interpreting that correctly.
I believe section 3.b and 3.c are the rights this is referring to, where you can request the source, and even be changed for the physical act. Suggesting this extends to the license of the implementation of their contact system doesn't make sense. No method of contact, except physical, is going to meet your requirements, including sending postage, where the software used to sort your mail is not GPL.
I believe section 3.b and 3.c are the rights this is referring to, where you can request the source, and even be changed for the physical act. Suggesting this extends to the license of the implementation of their contact system doesn't make sense. No method of contact, except physical, is going to meet your requirements, including sending postage, where the software used to sort your mail is not GPL.