You ask a good question, and to be honest: I don't have a good answer to it yet. I'm still trying to work out a consistent position on the whole topic. Therefore I'm going to give you a rather tentative view.
I currently feel there's a spectrum of cultural importance on media. On the one end, you have your sonnet, that few people even know exists. On the other end, you have Game of Thrones.
At some point a piece of media becomes so widespread that it becomes a part of shared culture. It could meaningfully affect everyone[0], like Harry Potter or even Game of Thrones (I'd expect everyone to be at least aware of its existence). Or a subculture, like Star Trek or Star Wars for people with even passing interest in science fiction (or STEM), or StarCraft for people into PC video games. These works weave themselves into the fabric of society - people lift phrases from them, make metaphors referencing them. Hogwarts is the modern Troy. Being unable to access these works - especially at the same time everyone else around you is perusing them and talking about them - becomes cultural isolation.
With books, we have it solved, thanks to libraries. Almost anyone can get legal access to almost any written work - where they don't have the money, they can substitute it with effort, to get to a library (and possibly convince the library to buy the work, or fetch it through inter-library loan, etc.).
With music and movies, we had it solved in the past too. You could be exposed to popular works through radio and TV, both private and public. This worked better pre-Internet, because while media distribution was regional, the culture was also regional. Not so today - with the spread of the Internet, the culture became international almost overnight. With Internet, enough of us in Poland can enjoy Game of Thrones simultaneously with the rest of the world that it becomes expected of everyone to enjoy it in lockstep with the whole planet.
Videogames gained mainstream popularity around the same time Internet did, so there was never an equivalent of a library, or public broadcasting, for games. You either buy one or pirate it. If a game suddenly gains worldwide popularity[1] and every one of your friends is playing it, you'll either buy it or pirate it, just so you can feel included in their conversation.
And that's ultimately the main justifying factor I'm currently exploring: entertainment media are social objects. One of their most important roles is to glue communities together, by giving people shared experiences to talk about. Once a work becomes popular enough, I can't really blame people who pirate it because they too want to be a part of the conversation.
The sonnet from your example? It isn't popular at all, it isn't part of culture, so I'd consider pirating it as an act of malice. My view would change if I suddenly started seeing it mentioned on the news.
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[0] - Not everyone everyone, as there are plenty of people with little to no access to Western media. It's much less than 50% of the planet, but much more than "the West", so I don't feel writing the latter would be right either - so where I say "everyone" in this thread, it is to mean "everyone within the broadly understood sphere of influence of the Western culture".
[1] - Which doesn't happen by accident; a lot of money and effort goes into marketing, in order to make this happen. And piracy was historically a part of it, too: few people actually bought originals of Diablo or StarCraft or Red Alert in Central and Eastern Europe, and yet they were hugely influential there, all thanks to folks with CD burners in their trucks.