I don't know exactly why it's like that but it's the same bug for preventing the siege units from moving without roads/ships/airports and preventing them from gaining a v status if built on a city with barracks.
What if the siege units didn't have a v status in the future games but cannons and perhaps even catapults would have a city buster flag on? Meaning that they would be 100% more efficient (like artillery and howizers already are) when attacking against a city? This would be a big change to the ruleset so it would need to be discussed about.
Is there a summary about how governments work? The help/manual isn't all that revealing.
The ruleset of LT33 is mostly similar to LT31. You can find very detailed summary here (including governments):