I hardly ever see anyone fire on a plane that is obviously completely disabled and going down. So I guess you guys are complaining about someone stealing the kill on the plane that you plan to kill sometime in the future... if things go well. In other words, you expect everyone else to know which plane you've picked out for destruction and to stay away until you finish your task.
OK. Maybe that would be obvious if you were hard on its tail. But, if you are not closing on the target, and the undamaged target is running, and I can catch the target, then I'm going to steal "your" kill.
Or, if "your" target appears to be on your six and you haven't waved off help via range vox, then I'm going to engage to try to help you.
Or if your target is smoking but he's still going to make it to take out some strat target, I'm going to try to stop him.
Or, if you are flying an A20 or TBM, then people might often steal "your" kill because they think you are in trouble and are trying to help you. How are they to know you are not a bomber in trouble, but are some silly person who likes to show off by getting kills in low performance planes?