Basically every country agrees not to fuck with each other's stuff. The cost of getting mass to orbit and the rather limited intel that little robot can gather makes it work.

Well before we see Moonraker level satellite theft, we have to contend with the much cheaper "let's just shoot them down" option. This capability is currently only within the reach of countries that already have an active space program or are aligned with a country that does. The worry is if two smaller nations decide to try it to get the upper hand in a conflict.

If it's a military or spy satellite, it most likely is equpped with a small explosive charge. It's better to blow it up than have some other country looking over the technology. It also prevents it from being recovered should it ever fall back through the atmosphere.