(governments) aren't obligated to enforce every law on the book, they are usually content just HAVING the law on the book
This is simply evil and, in my personal opinion, flagrantly immoral. In my book it completely undermines a government's legitimacy.
The first country to make "the state failed to enforce it uniformly" a valid defense against charges of breaking a law will win at least a few wealthy immigrants happy to pay lots of tax. Without "failure to enforce" as a defense, the legislature has no incentive to limit the scope of the law.
If the public prosecutor's budget exceeds the public defender's budget, the court is illegitimate.
If private citizens cannot file for criminal indictment (at their own expense), the court is illegitimate.