Well.. two things here. First, if you don't like it then destroy the tank. Secondly, I'd say that they are doing good since an engineer has to be occupied by repairing the building and can't go do anything else. The engineer is also building points here, although not as fast as the tank.

As far as snipers go.. why does every other person want to be a sniper and kill everyone? I think you do actually get more points if it is a special unit. Obviously killing the free ones doesn't get you much, but it's the same for regular infantry who are taking more of a risk by running through active combat areas.

One thing I think needs to be changed is that repairing vehicles should provide points to engineers, since they do so much of it without being rewarded.