Allowing a department to keep revenue that they obtain from doing their job gives them an incentive to try to get more revenue. This is the exact same reason that many people who work in sales receive bonuses for the number of things they sell. There is no reason that law enforcement should literally profit from dishonest work.
I have no idea why anybody would even think law enforcement officials have any right to keep a percentage of the drug money they seize. They're just doing their job. Just because they happen to stumble across a situation where they find a lot of money doesn't give them any right to keep it for themselves.
By giving the seized and forfeited funds to the organization as opposed to the official, the organization as a whole benefits with new equipment, better facilities, and by correlation, better performance by the officials. Also, with more money coming in to the organization through seizures, the budget rises and officers could receive raises. If a percentage of the money were to go to law enforcement officials, the officials could become motivated for the wrong reasons and only be looking for money to seize rather than bigger problems (i.e. drugs, illegal guns). Once the law enforcement organization has the forfeited money, they could have guidelines that lay out how they can spend it, and a bonus for the officer(s) involved with seizing the money could be included with those guidelines.
I think that law enforcement officials should be paid much better than they are. It's human nature to want to take available money, like with drug busts. I do think there should maybe be some formula to allow them to benefit from what they seize if the money is illegal. But the shouldn't just be able to take it, I think that would encourage some not so savory activities and maybe lead them to be less objective when it comes to evidence.