Now, there are fanfic authors who think it's unfair when someone suddenly starts policing their copyright because 'everyone else does it and you never went after them!'

The real reason it's unfair is that copyright violation doesn't have any punitive component, so you shouldn't be able to sue people offering things for free (unless they're actually replacing the market for your own work) -- but they put in these ridiculously high 'statutory damages' (up to $100,000), so you can. Yay.