The percentage-of-profits argument seems good on its face, but let's look further.
First, let's clarify: it's a percentage of sales. Percentage of profits would require that they expose a balance sheet. Movie studios are infamous for finding ways to pad expenses, so percentage-of-sales is an act of good faith, right out the gate. It also implies a greater business distance between the self-publishing platform and the author.
Next, let's look at details.
Traditional publishers take 92% of the paperback gross (retail sales price). Some of that 92% goes to the distributors and bookstores, but that's typically about 40% on paperback, meaning that the publisher is still taking 6/7 of the wholesale price. (Not all publishers do; Baen (a niche publisher) takes 80% and a few take less. My source is back articles on madgeniusclub.com .)
They justify this by the risk that they take by paying an advance, but that risk means that they have the authority to demand any change they want in the book, or to refuse the book altogether. The author no longer controls the content. In exercising control, they make themselves gatekeepers.
The Amazon self-publishing platform takes 30% of the gross. (I don't have the number for e-books handy.) What does that 30% pay for? It pays for print runs (print-on-demand), an expense that recurs with each run of books. It pays for a retail cut for books that go though distributors to bookstores. (Yes, if you've bought an ISBN, your book appears in the catalogs of Ingram et al.) It pays for bookkeeping, it pays for fulfillment (handling and shipping the order). It does not pay for a professional 'editor' whose word overrides yours. It does not actually pay for book formatting, since you deliver (not submit!) the MS in a digital format suitable for the process. If the layout is bad, that's your problem, and you can correct it on subsequent print runs/ebook sales.
Amazon has paid no advance, has invested no money 'on spec'. Their risks are limited to a few cents of digital storage and the cost of each PoD run--minimal inventory costs.
The same thing applies, with minor differences (percentages and fulfillment processes) for the other self-pub platforms.
It wouldn't surprise me that these facts, thus presented, would by themselves convince a court.
Nor would it surprise me that the First Amendment/gatekeeper argument by itself would convince a court--which means 'convince a judge'.
Now, as the the question of 'renting a press' versus paying for a service. There are things that cannot be rented without skilled operators, unless you are one yourself. Need a crane? You can hire (rent) one, with operator, transporter and driver (if necessary), and a crew of riggers. OTOH, if you are a lifting/rigging contractor and one of your cranes is down for maintenance, you can rent one from a specialty supplier and supply your own crew and transporter.
If an engineer calls for an A/C unit (for example) to be placed on a roof, it is the engineer's duty to be sure the roof can carry the load. Unless the roof is obviously rotten, the crane owner (contractor) will not be blamed if the A/C unit falls through the roof a week later. The contractor does not have the expertise to know, unless there is an obvious deficiency.
What would the equivalent be in Amazon's case? Maybe a book that is part of a specific criminal conspiracy. MAYBE. And maybe not. If a bank robber takes a bus or a taxi to the scene of his crime, the bus or taxi operator/owner will not be held guilty for transporting him. Even if he boasts of what he is going to do, the most they could be charged with is a failure to notify the police, which would at the most make them an accessory before the fact. It's hard to imagine a criminal jury convicting someone on that charge, especially if there was so little time it could have made no difference. (Civil court is more dicey.)