It seems strange that a neutral mine isn't automatically captured, while an enemy mine is. It's just very strange.

The implication is that while a mine is neutral, no one is working there, but as soon as it's captured, continuous work begins for the player who owns the control point. I don't think that's strange.

Added after 14 minutes
This is something like a holding system, where, figuratively speaking, a city or fortress is the holding company, and resource sources are subsidiary companies. According to this scheme, capturing the parent organization automatically leads to the subordination of the previously controlled subsidiaries.