Microsoft Urban Assault supports the completely free mouse cursor movement, which is used to interact with various command interface on the screen, or perform various actions which is a crucial feature for the RTS-style functionality.
By default, vehicle controls in the FPS-mode are usually accomplished via either a keyboard or a joystick (as recommended by the developers). But the player can always decide to activate/deactivate the 'Mouse Control Mode' at any time by clicking the right-mouse button while occupying a vehicle in order to control it with both mouse and keyboard, or mouse and joystick inputs simultaneously in the first-person control mode.
This function allows for more precise and reactive FPS controls in vehicles for the player, but sacrifices some (or most) control over RTS elements since the mouse movements become directly locked to the first-person view of the vehicle. Therefore, the player should make prudent decisions when they should activate/deactivate the mouse control mode based on the situations and tactical/strategic needs (if they intend to use it).
The simultaneous input that becomes possible with mouse control mode has an additional benefit, where the players can turn their vehicles faster than usual in the first-person mode by applying both Mouse+Keyboard or Mouse+Joystick inputs together. Apparently, this is not a bug considering there is a game document referencing this feature.