> Why's it treated as being a "per page" subscription? Doesn't that favour subscribers that use the printer to produce full-page, full-colour images at maximum saturation? Doesn't it disadvantage people who want a page of text?

Because the cost of page of text is so neglible in terms of ink that any limit that would let them upsell people printing text pages would be hit nearly instantly by people printing photos, and there's a lot more of the former group. It also lets them sell the potential that you'll print some colour photos at some point, when you won't.

> Why's it treated as being a "per page" subscription?

Because that's easy to reason about. A page is a page, no worrying "ok, this document has 10 pages, but there are some big graphics, what percentage of the ink is that using".

> Doesn't that favour subscribers that use the printer to produce full-page, full-colour images at maximum saturation? Doesn't it disadvantage people who want a page of text?

Yes it does. Unsurprisingly, people in the first group are more likely to like and use this model, instead of buying ink cartridges.

>Why's it treated as being a "per page" subscription?

Because that's the business model. Consumers care about the ability to print pages. They don't actually care about ink. As a consumer it's easy to see a subscription plan of 10 pages a month rather than 0.01 gallons of ink a month. The people that are disadvantaged are people who buy more pages than they use. This is offset by having unused prints rollover into the next month, but if you never print them then you have wasted that money.