Its upsetting to people because to them, its no longer a game system/AI that they're playing against. Its a person that's cognizant of the fact that they are having their fun at the expense of yours. You can say that it makes no difference, but once a player is aware that someone else is trying to ruin their experience and forcing something they don't want on them - You're going to get a negative response. Against AI, they have no one to blame but their own play. They didn't manage to "beat the game" as it were. Against a person - Its different. They're going to lay all the blame on the pirate, because well... You're accountable and the game's AI isn't. It has no personality to speak of, so it can't be a target for frustration. When there's another person on the other side, it becomes personal.
This is what I mean by anger and bitterness, not everyone sees it the same way as you and it isn't just split into "pirates and normal people".
I'm just going to start by highlighting that I am more of a care bear than you you will ever be, I will tell you that now. In Eve I farmed and played the markets enough to own supercapitals, despite never using them in combat. If you don't believe me, check the forums and you will see a character called "Simi Kusoni" buying supercaps and supercap characters for the 3 person corp "Hello Kitty Fanclub". Funnily enough that wasn't much of a PvP corp (although I did kill people if they got in the way of me making money or accomplishing a goal).
Now at this stage you will undoubtedly go "hurr durr go play Eve then, this isn't the game for you", as I have seen that response countless times on these forums, but the truth is I think this game has potential. It is rare to see new multi-player (I draw short of calling ED an MMO) sandbox games as they're difficult to balance, and the flight mechanics are amazing in ED, they're great fun and I'd rather not give up on it so easily.
So yes, solo exists and should continue to exist. But the challenge in games like these? The "emergent gameplay" that gives modern sandbox games their long life and allows them to feel like a living, breathing universe is provided not by NPCs and scripted events but by multi-player interaction.
Your solution is to force everyone into solo for farming purposes, pirates included, and that isn't going to create the "living breathing universe" that FD intended. Those kind of worlds aren't constructed out of badly hashed together NPC scripts and mission arcs, but by introducing a reason for player interaction and mechanics by which those interactions can gain some kind of structure.
Eventually in games like that achieving great wealth (or whatever else your personal ambitions are) becomes a satisfying goal because you've achieved it despite great obstacles, not some lousy AI you can fly circles around but real people with access to the same resources as yourself.
I don't really have any solutions, FD are in a sticky position when it comes to action they can take on the game. But I think the above is a fair vision for the open aspect of the game, and I don't think NPC piracy really adds anything of worth other than another meaningless grind that makes the game even more dull for the PvE pilots among us who enjoy a challenge.