They're different situations. One failed multiple drug tests in the UFC, the other is RVD who no one cares about. The optics are bad with Riddle, even though he's gotten insanely good insanely fast. WWE didn't want the negative attachment at the time.
But they're WWE. They have the luxury of not having to sign everyone in the world, they're going to be more than fine without him. They're in the position of being able to pick and choose, and that's what they're gonna do. While Riddle is great, there are lots of other who are also great and without his baggage. They can just as easily sign them, and that's what they'll do.
It's still a massive mistake, but that's how it is.
Most don't know who Matt Riddle is either to be fair. I agree they will be fine w/o him but it's a weak excuse either way.