Mostly because it's a game where we've already seen our main guy get pulled into multiple peoples messes. Not to mention the story possibilities could go in any direction. How the MC could help, again, could go in many ways. Whether on the DIK/Chick path you still help people along the way. Quinn is surely getting in over her head and i'm guessing at some point we as the player will get sucked into it and will likely get a choice as to how we help. Quinn clearly has affection for the MC so if she's had other avenues closed off such as help from Tommy, she can't get help from Sage with what she's been up to but the MC knows about both the drugs (To a lesser extent) and the pay for girls scheme she's running whether you've indulged in it or not, you still learn of it in game.
Perhaps your right and it won't be a thing but i think Quinn will certainly be in deep shit at some point and who knows, maybe she'll be the "AL" sacrificial lamb! lol (Hope not) And to your point of thinking it should be optional - I would likely think there will be some choice there, much like the Hell Week tasks where you can pass the buck on to Derek more, you know, if you want to be a complete pussy ....
But i understand some people might not want to be forced into something like helping Quinn. She seems to be the Marmite of this game .... Love her or hate her. Very little to no middle ground.