You don't have to. But it goes beyond dislike, some people even try to invalidate their Euro 2004 win because of their play style. They don't play "right"
It's fucking fear is what it is. Even now, of course Greece is a longshot. As a fan, I can't say with a serious face that I expect them to beat Costa Rica.
But those European team fans especially remember Euro 2004, they remember that series of 1-0 wins for Greece in the 2nd round over teams they had no business beating (France, Czech Republic, Portugal).
What's worse than watching a style of football you disapprove of? Having your beloved team lose to it. That is where the hatred comes from. If Greece played defensively and did what they are expected to do, lose, nobody would care.