> Liberal democracies usually operate under the principle that you can ask anything you want of someone
Except in cases like blackmail, extortion, racketeering... even harassment - you won't get much luck asserting the person you were harassing never bothered to tell you to stop. There is a whole host of behaviors that do not require the victim to mount a defense, because we've identified a power imbalance that might make it infeasible.
I think you're missing the problem with the current situation. It's a matter of incentive, and the fact that the FBI has no incentive to stay within the parameters of the law, no incentive to limit its requests, no incentive to get things "right". When a mere mortal citizen steps outside the bounds of the law, they are swiftly punished, and ignorance is no excuse. When anointed government officials overstep the boundary, it's no big deal because quite often, there's just no mechanism to punish them for it. But there needs to be such a punishment, otherwise trust in the system erodes rapidly.
The dilemma you posed is a false one. There is plenty of nuance inbetween the two extremes - "Don't let the FBI ask" and "The FBI can ask about anything and it's your job to figure out if it's legal or not". Possible recourse could require a pattern of abuse, determination of gross negligence, the punishment could be contextualized - mistakes leading to fines in favor of the citizen, while gross abuse leading to criminal charges. There's plenty of room for discussion of the specifics of such a system, but it must exist, and also the public must see it applied effectively in cases of extreme abuse.
I think your comparison of "you can ask anything of someone" ignores the reality of the relationship between two private citizens, and a private citizen and the government. The government expects trust, demands obedience and respect, but you cannot have any of those if it will also bend the rules it is supposed to be following, lie and cheat its citizens out of their rights. Even if staying within the marginal bounds of the law, this creates an adversarial relationship between citizens and the government, and breeds animosity and contempt. I believe there is a middle ground to be found between the government being paralyzed to act due to fear of making mistakes, and citizens living in distrust and fear of the government, which is supposed to serve them.
This "feature" of the legal system, where the weak are trampled because they do not have the power to assert their rights, makes it rotten to the core. Ask any CDL what the outcome of this "feature" is, for the poor, the uneducated, the ignorant, for minorities and for the weak. The law does not need to protect the strong - they already have the power to do so themselves. If a legal system fails to protect those weak in society from abuse, then it is at best pointless, and at worse, just another tool for those in power to subjugate the rest.