just that the act of downloading a file is not magically illegal despite no law defining it so
Here are the laws that make unauthorized downloading of copyrighted works prima facie illegal in the US:
17 USC 501(a): "Anyone who violates any of the exclusive rights of the copyright owner as provided by sections 106 through 122 ... is an infringer of the copyright ... of the author."
17 USC 106: "Subject to sections 107 through 122, the owner of copyright under this title has the exclusive rights ... to reproduce the copyrighted work in copies."
17 USC 101: "'Copies' are material objects ... in which a work is fixed by any method now known or later developed, and from which the work can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. The term 'copies' includes the material object ... in which the work is first fixed."
"A 'device', 'machine', or 'process' is one now known or later developed."
"A work is 'fixed' in a tangible medium of expression when its embodiment in a copy or phonorecord, by or under the authority of the author, is sufficiently permanent or stable to permit it to be perceived, reproduced, or otherwise communicated for a period of more than transitory duration. A work consisting of sounds, images, or both, that are being transmitted, is 'fixed' for purposes of this title if a fixation of the work is being made simultaneously with its transmission."
If Alice has a file server on which are copyrighted works, and Bob, without permission from the copyright holder, downloads them, Bob causes his computer to fix those works in a tangible medium of expression (such as a hard drive), which creates new copies of those works. The copy is the tangible medium, again e.g. a hard drive, not the mere intangible files. By creating copies without permission, Bob has infringed on the exclusive right of the copyright holder to make new copies.
So, it's prima facie infringing.
You actually conceded this point earlier; you obliquely referred to 17 USC 117, which is an exception dealing with computer programs. Section 117 is completely unnecessary if no prima facie infringement occurs. Much in the way that you don't have to bother raising a defense to a charge of murder, like self-defense, if the supposed victim is still alive. Or if programming is more your thing, think of an if-then-else statement: if infringement occurs, then see if section 117 applies, else infringement has not occurred, so exit.
So you appear to agree that downloading is prima facie infringement, the question is simply whether the exception in 117 saves the downloader. It almost never will.
17 USC 117: "(a) Notwithstanding the provisions of section 106, it is not an infringement for the owner of a copy of a computer program to make or authorize the making of another copy or adaptation of that computer program provided:
(1) that such a new copy or adaptation is created as an essential step in the utilization of the computer program in conjunction with a machine and that it is used in no other manner, or
(2) that such new copy or adaptation is for archival purposes only and that all archival copies are destroyed in the event that continued possession of the computer program should cease to be rightful.
(b) Any exact copies prepared in accordance with the provisions of this section may be leased, sold, or otherwise transferred, along with the copy from which such copies were prepared, only as part of the lease, sale, or other transfer of all rights in the program. Adaptations so prepared may be transferred only with the authorization of the copyright owner."
17 USC 101: "A 'computer program' is a set of statements or instructions to be used directly or indirectly in a computer in order to bring about a certain result."
This usually won't work because Bob, the downloader, almost certainly 1) isn't the owner of a copy prior to making a new copy, which is necessary in 117 to be allowed to make the new copy; 2) isn't making copies for archival purposes only; 3) most data, such as music, movies, etc. will not be treated as computer programs by courts, despite the broad language in section 101.
I mean, feel free to try it, but you have my guarantee that you'll not just lose, but be laughed at.
The theory that when you download you cause a copy to be made is erroneous because there are specifically outlined situations in law where a transfer in that way is legal and the onus rests on the server to be compliant- not the downloader.
Okay genius, I've cited the relevant statutes for making my argument. It's time for you to put up or shut up.
Remember to cite very specifically, and to look at definitions. You may also want to look at relevant caselaw.