Whats to stop someone from posting up near the objective and just mindlessly killing anyone who tries to walk near it? That would be mindless killing wouldn't it?
Nothing to stop them, but even that requires more thought than the same strategy in deathmatch would. In deathmatch, the enemy is going to come from spawn and go for kills. If you set up facing the general direction of their spawn and kill anything that moves, you can win.
In objective games, the enemy is focusing on the objective, a good team wont care about getting single kills, and will take the time to get into position and kill most of the enemy team in one attack, then move in and work on the objective, as defense, you have to try and predict what they are going to do based on limited information, and prepare for it, pick a good position to stop it, and coordinate with your team who are trying to do the same. There's a lot more tactics, even if the end product seems similar.