the damage should stay,i don't know if anything even changed,but far as i remember,support spams are already terrible as they are. the moment your own mines won't hurt you,not only the attacking team would lose a major advantage (which is already a huge risk),but techs and shockers could mindlessly put them wherever they want with almost no penalty.
other then that,for your reasons,
reason A.other bombs are weapons,mines are traps. why wouldn't it be fair? it wouldn't make much of an offensive difference either if they wouldn't damage the user
B.by saying that,you encourge the fact that techs/shockers should simply place packs of mines on the control point while expecting 0 penalty. not only it shouldn't matter since people place suppresors and aegises on the roof,it just ruins the point of thinking more strategically with your mines.
C.then how does your solution help them? i could sit and think of other solutions but there's no chance in hell your solution would give mines more of an offensive use to the classes.
D.then make one,seriously,just draw the mines blue/yellow and give an indication for your team (and your team only).