Player A: Wants to expand to resource-rich area of board. They build two roads and plan to build a settlement and continue their road to the resource-rich area.
Player B: Also wants same location as player A for their next settlement. They build two roads, ending at same hex intersection as player A. Player B builds their settlement first.
Player A: Still wants resources further away, so places another road on the other side of player B's settlement.
Player A has a road going into the hex intersection, and argues that they can build a connecting road going out of the intersection, regardless of whether another player has a settlement there.
Is this really a legal move? If not, where in the rules does it specify that this it is not allowed?