Why cant bind "Next Target" to something else?

[11aTony]

So I wanted to bind next target to mouse 5 in order to avoid alt+tabing out of the game while flying and I realised no other button works. I unbinded next target and any other function I try to bind to tab it colours red (next target it says next to it). Except optics and maybe something like step over works on tab.
Speaking of "step over", I binded it on same button as "eject" (OEM 102 and 2x OEM 102 = "get out") and it colours red. It works but step over is red, despite its infantry only function and eject is everything but infantry.
So I could bind "free-look" to something else but really, that is no solution. Besides Im too used to alt.
Is there something I am not aware of or is this a bug?
Further more I have a request. Make lav25 and maybe zoom toggle. Its only toggle in rifle optics, tanks, air but not lav for some reason.

[LEEROY_UK]

I hate that ! just as your on a straffing run........desktop ! arrrg !