If an enemy nation wanted to conduct an EMP attack, they wouldn't need to do it with an ICBM that we'd be able to identify as such. Getting an object into LEO would not, on its own, stand out, and would provide an avenue for attack (or the threat of an attack) that would not be detectable in the same way as an ICBM launch.

Of course, the harm to their own systems would be substantial as well given that it would have a huge impact on satellites in orbit, not just systems on the ground.