The fact that sometimes people get the 30 minute penalty before they can join again, but sometimes they don't, sounds like a bug. Whatever the rule is, if it was working correctly, it would be consistent. And I'm pretty sure the rule is (as it should be) that if you leave you get the penalty for leaving.

The fact that you're able to start Duty Roulette any number of times and keep leaving while still having the Roulette bonus available for another try at it, is an exploit. It defeats the purpose of the Roulette reward while ruining the game for other players.

Both should be fixed. Roulette should get you one chance per day at a dungeon, and you should need to complete it if you want the reward for that day. And leaving any DF dungeon (whether Roulette or not) should give the delay-before-requeuing penalty.