Contact is not the test
A hook that has no effect on the puck carrier's stride and a hook that ends a clean breakaway can look nearly identical from a wide camera angle, yet officiating standards ask the referee to weigh the second far more heavily than the first. The rulebook cares about restraint of an opponent that changes the outcome of a play, not contact as an abstract wrong.
This is what frustrates viewers who expect a rulebook to be applied like a measuring tape: it is not measuring contact, it is judging consequence, and consequence requires a judgement call that a slow-motion replay of the contact alone does not settle.
Consistency over strictness
Players and coaches consistently say they would rather have a game called the same way from the first minute to the last than have it called strictly. A tightly officiated first period followed by a loosely officiated third feels like a different sport was played in each, and that inconsistency is what actually generates complaints, more than the overall level of contact allowed.
This is also why the standard is widely believed to loosen in the postseason: whether or not it genuinely does by rule, both teams behaving as if it will creates a self-fulfilling shift, because officials are also calibrating to what looks proportionate against what everyone on the ice has silently agreed to allow.
The calls that are not made
A referee who lets a marginal infraction go is not failing to officiate; he is making the judgement that intervening would have a bigger effect on the game than the infraction itself did. That is a real decision, made under pressure, and it is one of the hardest parts of the job to evaluate fairly after the fact, because the alternative — the penalty that was not called — is invisible to everyone but the official who chose not to raise an arm.
None of this argues officials get every one of those judgements right. It argues that the standard being applied is more coherent than reflexive by-the-letter enforcement of the rulebook, and that a lot of the sport's ongoing argument about officiating is really an argument about consequence, dressed up as an argument about contact.