Why Are Some Fans Critical of Fighters With Great Defense? ( Or "In defense of defense") )?

Boxing has been called the "sweet science" because it is a sport where hitting without getting hit in return is the cardinal rule. But great defensive boxers like Muhammad Ali, Willie Pep, Pernell Whitaker and now, Floyd Mayweather had or have, for most parts of their career, been criticized by some fans for their defense and ingenious capability to hit without getting hit. Why, former macho sluggers like Roberto Duran would not have taken his career farther, after having been kayoed face-first by Thomas Hearns and beaten by a slew of upstarts in the higher weights, had he not given more attention to defense. Even present-day boxing wonder Manny Pacquiao's handlers are now saying that Pacman has improved tremendously since he also put emphasis on defense ( "You can't hit what isn't there" has been their guiding principle since he moved up to face the bigger but slower guys in the jr. welter and welter ). So why the beef over Floyd's heavy emphasis on defense which has been his trademark ever since? Why is it wrong for them for Floyd to maintain this stance and right for Manny and his likes to improve their defense? Isn't that unfair?