Ehm, selective enforcement by the state most certainly can be a problem, I cannot, however, see how selective enforcement by people or companies (who, in civil law, are always free to sue or not to sue for whatever reasons) is bad or can even be considered selective enforcement in any way.

It’s a pretty central part of civil law, actually (making sure – at least in theory – that only if two parties absolutely cannot agree on their own, the legal system has to kick in), and I think it would be downright evil to require people to always sue everyone.

Even state attorneys can pick and chose (to some extent), but to avoid arbitrariness they have to have good reasons for what they do, who they decide to prosecute and not to prosecute. They can’t decide not to prosecute for arbitrary reasons. (I know that’s the case in Germany, I think that's the case in the US, please correct me if I'm wrong.)