Presumably it's an issue of national sovereignty - it is their country, and they are broadly able to do any activities on their land as they see fit, just as the EU and US have seen fit to develop nuclear weapons.

Presumably the alternative is to threaten to invade or impose economic sanctions, but that doesn't seem to me like the beginning of a productive conversation. If anything it might increase the urgency of getting weapons, of any type, to be taken seriously.