They aren't illegal.
They are tungsten rods with no explosives or anything.
They are entirly legal and follow those Cold War weapons agreements
I stated they are not illegal, as weapons of mass destruction are. But they do't actually serve much purpose, in truth. Conventional weapons are not illegal in space. Nuclear weapons are. However, if orbital bombardment weapons were to be developed, they would probably be made illegal in short order.
Check the wiki
And about anything else
Most things say you just have to drop it and it's good, or drop it with little effort
And ya it'll take a lot of energy to get it up there but it's just as effective as a tactical nuke and much less nuclear aftermathy
I would much rather kinetic bombardment be the weapon of world war three rather than nukes, much less if any fallout at all.
Unless you want it to drop several months later, you need something to push it out of orbit. It won't simply drop to Earth for the same reasons satellites don't need to be constantly firing thrusters. (Most satellites need to 'top up' their orbits every now and again because of drag from the very top reaches of Earth's atmosphere slows them down, I believe.) But you need something to push you out of orbit if you want to hit the ground any time soon. And satellites travel fast. That's a lot of momentum, especially for a big block of heavy metal.
In truth, a conventional bomb of the same size and weight as the tungsten rod would be just as effective. It would also not have the ridiculously huge cost of actually getting the bombardment satellite and it's ammunition into orbit in the first place.
It has a few advantages, compared to an ICBM, true, but it also has disadvantages. it would hit quicker than an ICBM, and so be harder to detect and intercept with anti-missile weaponry.
It does, however, have significant disadvantages, and these are no doubt why it has never passed the theoretical stage.
a) Huge cost - it costs one hell of a lot to get anything into space, never mind literally blocks of heavy metal.
b) The weapon is blind during reentry - a plasma sheath would efectively render it unable to track mobile targets, unlike an ICBM
c) A satellite can't dodge - if the satellite was discovered before it could launch it's rods, it could be destroyed easily. Satellites being destroyed is bad for everyone due to aforementioned Kessler Syndrome. We can already track tiny bits of debris in orbit - a weapons satellite would certainly be watched by every nation that had a telescope to put together.
d) The satellite has to pass over the target - there's a limited amount of places a satellite can cover at once. A rod has a fairly narrow area of effect - you couldn't, in fact, level "any city you wanted in fifteen minutes no matter what".
e) It'd probably be considered a war crime - it might not use nuclear materials, but neither does gas. If it was as powerful as you say, it effectively couldn't be used for the same reason nuclear weapons aren't. When you destroy a city, people tend to get upset. There's not enough precision in it to use it without civilian casualties that would probably be seen as unacceptable. There's civilian casualties, and then there's an entire city worth of people all at once. One is considered more acceptable than the other.
Modern warfare is about mobility. The entire British nuclear arsenal is on submarines currently... somewhere. So if you destroyed, say, Birmingham with one of these rods - you could probably expect a nuke in short order.
Wars aren't done between developed nation states anymore. MAD is still good and strong. It's why Russia could invade Ukraine - Ukraine gave it's nukes back to Russia when it became a sovereign nation in return for an agreement of protection by Russia, UK and USA. It ended up rather poorly for them, but if they'd kept their nukes it never would have happened.
Similarly, wars against insurgents require more precision. Precision can be easily delivered with aircraft munitions and cruise missiles for a fraction of the cost of an orbital bombardment weapon.
Much like mechs, a cool concept, but there's a reason why they aren't a thing.