Spacetime is a continuum. It is Mathematically proven beyond doubt that spacetime is a manifold, which is to say, it appears locally “flat” near each point in the same way that, at small enough scales, a globe appears flat.
Spacetime is expanding. Which means the universe is growing. Time is expanding. We can think of “the present” as being the surface of an expanding bubble of time. Everything inside the bubble is the past. A fixed and unalterable part of the spacetime continuum. Outside the bubble is nothing. It is the future. It does not yet exist. Until the present expands into it it is just a theoretical possibility.
That is the easy part. Now consider time travel. We are already travelling into the future at the fastest rate possible. The rate of expansion of the continuum. We cannot get there any sooner than anyone else, because there is nothing there until everyone arrives. Literally nothing. So travelling to a distant future is not possible. There is no such thing. Yet.
The math of travel to the past is entirely different. It is possible, but only if it already happened. Therefore travel to the past involves always having done so, and allows no possibility of changing events historically. The attempt to assassinate Hitler always failed, we know exactly who was involved and what happened to them. What we don’t know is precisely which of the conspirators came from the future, when they came from how they got there, and how they insinuated themselves into German Military Intelligence without being discovered. That hasn’t happened yet. Or perhaps it has, and we don’t know. In reality, there seems to be no point in undertaking the attempt, because it failed, and always will. We know the coup will not work, but we also know they did try. So they always shall. There is no way that the attempt can be prevented because it already happened. At some point in the near future therefore, it shall probably be initiated.
That is putting it simply.
There is no need to police the laws of spacetime. They police themselves.
Heinlein was wrong. Vonnegut was right.