I think the preventing piracy would be for the multiplayer action (which is much more desired by pirates than the single player). Once you finish the single player campaign, nobody really looks back at it anymore (which is fine for people who don't like multiplayer StarCraft 2) but the majority buy SC2 for playing the multiplayer part of it.
Back in Brood War, there was the creation of the ICCUP system that uses its own private Battle.net server that bypasses CD Key checks making it so that anyone with some form of Brood War installation install it regardless of how they did it. That's their goal in terms of "prevention" (which did the job fairly well as well). Another huge factor was the KeSPA vs Blizzard scandal awhile back.
And if you get banned from SC2 because of the trainer, your other games are not affected. They only invalidate your SC2 license.
i think they stopped giving a crap around the time cata came out in my eyes. i just don't see why blizz would ban people from a game if they used a hack or trainer in single player which has no physical effect on multiplayer at all aside from a glowing blue number and a portrait or 2. it;s not like someone who sees me with 3000 score and a sarah kerrigan portrait is going to quit the game because of that he will assume that i played the story to the point of obsession. it's nothing but a number it's not going to give my tanks infinite range or anything like that if i cheat in multiplayer then ban me if i cheat in a story mission for sh*ts and giggles ignore me till someone whines that i had a 400 food army in their base before 10 min.