I think the biggest flaw is the location of the entertainment center next to sewer system.
You could move the piping, but you'd still have to reward the action. And if there's reward sensors there, clever animals will figure out how to trigger the reward without the action.
In other words, you can move the sewer but they'd probably just build another playground there.
Shouldn't lead to addiction, and would work as well.
I certainly wouldn't characterize urination as fun in the sense of "hm, how can I make this happen more often?"
I'd wager that's a non-ideal situation. Think about the times you've reached a completely full bladder...not a good position to be in.
So you have a combo. Taking a whiz is pleasurable, so you do it when you can. But if you get over full, you get a nice zap of discomfort to let you know you're in a danger zone.
It's basically how it works right now, no need for messing with positive reinforcement.