Nice one cam-op

I think the answer lies in the level of trust you have in those that have set the restriction and whether personal experience or knowledge has influenced it. Taking your example, what if the sea that day was like a mill pond and that you knew the area well enough to know that there were no rips? But you did specify lack of knowledge, so let's look at other things. Beach lifeguards are unlikely to have an interest in making such a restriction for any reason other than a genuine safety need and can probably be trusted to be experts on when and where it's really necessary. In that respect it's not a good analogy for speed limits which can be set by local councils against the advice of experts and for purely political reasons, and (not that I'm having a pop at you) are enforced by partnerships who have a financial interest. Nor are beach lifeguards likely to suffer from the same sort of inertia as governments and councils, and leave the restriction in place long after it ceased to be necessary.
More to the point, restrictions should never take the place of our responsibility for our own safety and our own judgement. Basically, do the red flags indicate the rest of the beach is safe? Obviously not - you can still drown there. Whether you know the beach or not you still have to exercise judgement about swimming in the unrestricted area, so regardless of your ability, knowledge of it, the conditions in the area, and lack of prior knowledge about that, it's still necessary to constantly evaluate the situation so that you know when to get out of the water. In that respect it's a bit like driving and treating the limit as a guide.