Learn Italian: Ne parliamo perché il decreto ha imposto che restassimo qui. - We are talking about it because the decree imposed that we stay here.

Because the verb expressing the command (ha imposto) is in a past tense. When you report an order or command made in the past, and there is a change of subject—from the decree (il decreto) to us (noi)—you use che followed by the imperfect subjunctive.

Could we avoid the subjunctive and just say "ha imposto di restare"?

Not exactly as written. The subject of the commanding verb is the decree (il decreto), but the people staying are us (noi). You could use an infinitive if you added an indirect object pronoun, like ci ha imposto di restare (it imposed on us to stay). But without the ci, you must use che restassimo to clarify who is supposed to stay.

The sentence starts with the present tense verb "parliamo". Doesn't that mean we should use the present subjunctive?

No, because the verb that actually triggers the subjunctive is ha imposto, not parliamo. The first clause (Ne parliamo) just gives the current context. Since the commanding verb itself is in the past (ha imposto), the dependent clause must follow the past tense sequence rules, requiring the imperfect subjunctive (restassimo).

As a quick refresher, what exactly does "ne" mean at the beginning of the sentence?