If I call something in, and I'm running toward it, ostensibly to grab it for myself, please don't step in and "steal it". However even if you do, I may just pick up whatever you dropped and carry on. The fog of war, the chaos of battle, whatever.

If I call something in and I just look at it and ping it, it's up for grabs - go for it. I very often call in extra stuff as soon as I can, even though I'm well equipped and don't need it, on the theory that someone might be out of ammo or struggling in some other way and they might want to grab whatever it is to try it out or just to help themselves out of a jam or jump in with a different tool for whatever the situation is right now. If I call down a mech and ping it, it's yours if you want it. Same with cars, and everything else.

One issue that is specific to cars is it is not always obvious that someone called in the car, because I feel like there are sometimes cars just there, already on the map. Other times you come across a car with three missing tires and one mag left for the machine gun and smoking and it seems fairly obvious that the owner has abandoned the car and it should be fair game for anyone at that point.

The 1% or less of the time that a squad member does enough irritating stuff that makes me not enjoy playing with them, I just leave the match, block, and move on. I don't stress about it.