I see, so you'd be fine with them scanning and storing a copy of every piece of mail that ever crossed your mailbox in the same fashion.
If you're ok with having to rely on their discretion we may already be talking past each other. A bedrock principle of the constitution and amendments are that the government's discretion can't be trusted, so discretion must be limited and controlled and standardized. My point is that discretion to use a power is no different from just having the power outright.
Appeal to tradition, a strawman, followed by an ad hominem. I hope you're saving a better counter for later. Parents don't need parallel construction, they already have absolute power and can investigate however they wish. Are you sure you understand what parallel construction is? A simplified example: A prosecutor wants to convict a citizen (for whatever reason) but they don't have any evidence, so they use illegal means to find evidence that they couldn't have practically found otherwise. But this is inadmissible in court, so they fabricate an "investigation" that -- through the prosecutor's superior "intuition" and uncannily accurate "hunches" -- ends up producing some of the same evidence, enough to make a conviction. I don't understand your goal here, are you saying that parallel construction is fine and the source of evidence doesn't matter? It sounds like you're trying to throw out the exclusionary rule altogether.
I'm sorry you're not aware about the existence of secret laws. Please read up on it [1], it may be enlightening. If you take issue with my use of the adjective "arbitrary", I'll give you that. (But if they're secret how is that really so different from arbitrary?) If you take issue with my colloquial use of "law" that includes regulations, executive branch pronouncements, and secret interpretations that fundamentally change the plain reading of statutes, I'm not sure what to tell you other than to stop being pedantic and think of the bigger picture. "Couterintel" is an overused excuse employed to dodge accountability, most of the time it doesn't need to be secret. "Subject to review". Yeah, sure [2].
What you see as "overactive imagination" I see as a healthy distrust of the government, because "all men having power ought to be distrusted to a certain degree" [James Madison, 3]. Yes the checks exist, but the systems I mention are the circumvention of these same checks. What I see from you is sticking your fingers in your ear, burying your head in the sand, and singing LALALA as loud as you can when presented with evidence of the very abuses for which you say we should stay vigilant.
[1]: http://www.brennancenter.org/publication/new-era-secret-law or https://www.nytimes.com/2016/10/18/opinion/the-governments-a...
[2]: https://www.theguardian.com/commentisfree/2013/jun/19/fisa-c...
[3]: http://avalon.law.yale.edu/18th_century/debates_711.asp