The nature of the weapon is such that when deployed, treaties won't matter much after that. Before that, it is hard to check what each satellite has on-board.

Also, EMP waves are not designed to kill people, so I am not sure what they classification as far as any treaties are concerned.

The level to which our infrastructure is vulnerable to EMP has dramatically increased since the 60s. That means, there is a much larger incentives to deploy such weapons now.