1. Implied - Not directly expressed.
Patent - A patent is an exclusive right to a particular invention.
License - permission to do what what otherwise be illegal.
IF a company gives you permission to do something that would otherwise violate patent law, you have a patent license for that task even if not patent was specifically mentioned. If Microsoft entered into a contract with me to manufacture android devices for their employees, that can't turn around a sue me for patent infringement of those specific enumerated devices on the purchase order. In the purchase contract it is implied Microsoft gives me permission to do those things reasonably required to fulfill the contract.
The is precedent I assume of using the principle of estoppel as a defense against patent claims. And estoppel may be implied. Ergo implications of representations made by a person may estopped patent claims against other persons.
2. The contract must be judged by it's actual language rather than ex-post-facto assertions of it's authors. It is the interpretation they would like to see in the courts, though not indicative of current law.
It may be true the FSF believes running a program is not a copyright issue, however there are precedents that claim loading a program into RAM constitutes a copying under the meaning or 106, especially if done by a licensee of a copy rather than an owner. A licensee being differentiated by having significant restrictions places on their ability to redistribute to transfer the copy of the work. The GPL does put significant limitations on this act, Section 4 specifically forbids you to "copy, modify, sublicense, or distribute the Program except as expressly provided under this License"
But this point is irrelevant. The GPL is not a copyright grant it is a license contract giving permission under certain conditions to do certain things that would otherwise be illegal. The GPL could have said that use of the program is not restricted by this license, but is simply says not restricted.
3. Section 8. allows the Original Copyright holder to expressly exclude distribution on such jurisdictional basis, why was this not done?
Anyways that's not quite what I think section 7 implies. If you agree not to distribute or use the program outside a license agreement, you may not longer distribute under the GPL unless royalty-free licenses are granted downstream. If there is a judicial ruling that the software infringes then you may not distribute it (under this license at least).
So with section 8 available (but not taken), then it must be presumed Ximpleware's distribution really did give a full GPL license to those receivers in the U.S., without additional restrictions. Now if the programs was distributed in violation of the GPL then it could be considered and inducement to infringe.
Having transmitted the program under a license in which no restrictions are placed on use of the program, Ximpleware is estopped from asking for judicial relief that would be a restriction on the use of the program.
Even in the case of the accusation patent infringement the GPL the GPL makes no termination of right.. The GPL just clarifies that distribution under the GPL may only only happen when in full compliance with the GPL; if for some reason you are legally unable to fully comply then you must forgo distribution.
If a third party had written the parser, then Ximpleware could claim any distribution in the U.S was illegal. However Ximpleware as the patent holder needs no patent license to distribute it's parser and may give licence to whomever it wishes. And section 6 of the GPL makes it clear upon distribution the person receiving the program also receives the license under the GPL from the original licencor. Upon the third party receiving the license from Ximpleware, they to have the implied patent license from the no restrictions on use clause. To distribute under the the GPL is to represent that you are imposing no restrictions on the use of the program.