> It makes perfect sense for the government to uphold agreements it has made.

The key question is what is actually in the agreement. 'The state will maintain the fence' doesn't indicate the level of maintenance or the timeliness of said maintenance, it just means the neighbor isn't allowed to do it.

> Secondly - if they did own the fence line, it'd be on the government to come and fix it when one of their trees breaks it, surely?

From discussions I've seen, generally there's no legal responsibility for tree falls, unless there's neglegence like ignoring a clearly failing tree. But leaving a forest to do whatever the forest wants doesn't feel like neglegence.