Why is this such a problem? Let me demonstrate the effects of this action through simple diagram. Red player (Red Circle) stands by his own TNT (Red Box), when he does this, he puts himself into the enemy's range/line of fire (Blue Rhombus). As such, when this happens, the said blue player (Blue Circle) could place a TNT beside the red player's TNT thus killing the Red Player in the process.
When players make this kind of mistake, it shows that they still need to focus and practice on keeping their distance from opponents. In a real world fight, much like a CTF duel, excellent fighters/players will intentionally keep some distance apart in order to get away from a 'line of fire'.
2. Jumping right next to their own TNT.
This problem is similar to the first one mentioned above. Players place themselves into the line of fire by jumping next to their TNT.
