General Order 7 specifically refers to Talos IV.
There are two problems: first is the possibility that anyone who approaches will be fooled by the mental illusions the Talosians can generate. Such an individual could conceivably remove a Talosian from the planet and never realize it.
The other, and more insidious danger, comes from what the Keeper told Pike: that humans could learn their power of illusion. Since the power of illusion was largely responsible for the decayed state of their society, there is a perceived significant social risk.
That's also why the general order doesn't say why it's forbidden to approach the planet -- the framers didn't even want that temptation generally available.
Presumably, if another, similarly dangerous planet were discovered, Starfleet might generalize the order by including a reference to a list of planets that may not be approached for any reason. Perhaps that has not yet happend; there may well be planets that are dangerous to approach, but not absolutely forbidden.
General Orders are very broad in scope, and significantly influence the decisions of Starfleet personel. I could believe that, 70-80 years into space flight, there would be only six of them. (Remember that Pike's visit, which led to the General Order, occurred some years before the events of "The Menagerie".) These are the things, like the Prime Directive, that you Don't Want The Crew To Ever Forget. You achieve that best by not going overboard. The rest of it is covered by procedures, special orders, directives, and suchlike.