I have never attacked another players 'target' trying to steal reps or cargo. I have however done so trying to fulfill quest requirements. If I see a marauder I need on the horizon, and I high tail it there, to find another ship attacking it (seems to show the enemy ships way before it shows the pirate/player ships). Then I will usually take only one or two shots because I want a 'kill' not reps or cargo, just trying to get credit for my quest.
Only other situation I come across regarding this is, I often sail off of PDF, and have watched ships shooting a navy ship for example, I engage a nearby EITC ship, and before sinking thiers, they start shooting at mine.. well i usually take a shot at thiers too... I mean its only fair..
Zeppers lvl40
http://pirates.teamofx.us