So the origin of the gun salute was to demonstrate your guns were empty. This doesn't apply to modern or energy weapons at all, of course, and the gun salute's meaning has changed since then. I have to think that starports have superb shielding or defensive batteries or both. The traveller combat system is certainly favors quantity over quality of fire, and so most small ships are easily out-gunned by a shore battery with no 1 per 100 tons limitation, and so would be foolish to start trouble at a starport.
Also, since the starport is Imperial territory, starting trouble will draw a Naval response, which is not a long term survival strategy.