It wasn't illegal because it didn't actually spike him on his head. It's sorta like throwing a soccer kick or stomp that misses. A move is only illegal if it actually lands. Intent doesn't seem to matter much except for low blows and eye pokes.
He could have let go so the fault lies with Guillard.
Yep. Varner isn't intentionally spiking, he is just falling down. Guillard just has to let go of his hold to escape danger.
From my understanding it would be illigal if Melvin would have landed with his head first. Which would be kind of a BS rule to beginn with.
Even the Rustam suplex action was dangerous no clue how to solve this though.
He could have let go so the fault lies with Guillard.
This^It's only illegal if Melvin didn't have the ability to release his hold. Notice that Melvin is holding onto Varner, not the other way around. Same goes for spiking your way out of an armbar. All Melvin had to do was let go.
Has anybody ever been warned about throwing an opponent right on it's head?