I don't even consider that an issue. It's not like he's abusing some bug; he simply has many members accounts. Unless they are acquired illegally, he's not doing anything wrong in my opinion. It's Jagex's responsibility to maintain a balanced prize pool (i.e. the number of items in the pool should be proportional to the number of participants so that people aren't statistically missing out). There are several ways to go about this.
Sure, he can use all those DTDs to farm gold for his main, but conversely, the money he spent paying for members (assuming it was done legitimately), could have earned him a fortune on his main too. If he actually makes a net profit, which I personally doubt, Jagex obviously needs to look at balancing prizes (ie not handing out 3 darts, 200m cash stacks etc) or balancing the drop tables.