Whether you agree that taunting should be legal or not... Giannis was already on a tech and the league sent out a memo saying this particular type of move after a Dunk would be enforced. Why even give the ref the opportunity? He taunted, it's clear as day per the league office's memo.
There's a reason you t up a taunt, especially when its something like Giannis bowing up/flexing on a guy. If you don't call it there, the same people calling the league soft for this, will be complaining that the refs lost control when there is a hard foul the other direction in retaliation.
They aren't allowing them, they are directing them. They sent a league wide memo saying taunting after dunks is a point of emphasis for officials.