You are jumping to conclusions. For one thing Tommy used his DIK point in this and so we don't know the full details of what it was about. Considering the plot it should have something to do with the Quinn/Vinny/Lily/Riona/drugs situation, but even that much is unconfirmed. I don't think Tommy would have spent his DIK point on his next few hits especially when he is going to pay it back. At the very least that is what he intends. Nick may believe that it was just Tommy feeding his habit, but considering the demonstration we have of Nick's thought processes and how he hero worships Vinny of all people, it's practically guaranteed that Nick is wrong and Tommy was doing something a lot more important. Rusty knows Tommy's reasons and since we know that he has previously refused to sponsor Tommy's life style, we can assume that Tommy wasn't just taking money to feed his habit.
The DIKs really should have a brotherhood meeting about what kind of limitations the DIK points should have, but whatever they end up deciding about them Rusty isn't a bad leader for sticking to the rules as they are at that time. Even if Tommy abused the DIK point system, he won't be able to do it a second time because he has spent his point.