"The fact is that U.S. law enforcement is bound by law (and the Constitution in particular) regardless of whether one is sympathetic to them, and regardless of whether or not legal standards are violated."
However, that's clearly not the only thing that they're bound by: there's an evolving discourse of law, and it's especially evident in the FBI lawyer's statement that many in the law enforcement community have a fundamental conflict - that they want to be Judge Dredd, not Sergeant Joe Friday. Law enforcement officers are bound by multiple and conflicting directives, and that means that they have no choice but to choose, to exercise discretion about which of those is most important. That's normal for most professions, but I argue that we should hold to far higher standards the people who get to legally use force.
My favorite example is micro-scale: speeding isn't actually very dangerous among the things you can get ticketed for while driving, but it makes up the lion's share of the tickets handed out because it's easy to enforce. You can point to a specific number from a mechanical device and say "there: that proves that this person was speeding." Other driving behaviors, often more dangerous, can't be so directly quantified, and so are more of a pain in the ass to enforce - and are enforced less. The conflicting directives result in a sub-optimal outcome.
Law enforcement officers in the US are not bad people: they're normal people, like you'd expect. The problem is that when you give normal people power without accountability, evil results. As the FBI shows in the linked article, they vigorously resist any effort at making them accountable - which is why it's so critical to heap on the accountability. Further, bringing me back to agreeing with you, the standards that they're bound by and accountable to, should be as clear as possible. The fuzz in the directives is part of the expanding power of law enforcement agencies, and we should acknowledge and actively work to counter that tendency towards expanding power without expanding accountability.