I can not speak for the habits of Greece. I do not know and am unwilling to put out the effort of informing myself as it is not important to me nor does it alter what I am saying. However, avoiding taxes is legal and is the fiscally responsible thing to do. It is the responsibility of businesses and citizens to pay the least amount of taxes that they are legally obligated to pay. Tax avoidance is what you are meant to do and is well within the law in any country where I have earned money.
Tax evasion is illegal and is morally reprehensible even if you do not agree with the manner in which your taxes are spent or feel that you are being taxed more than is proper. Tax evasion should not be tolerated and, if this is what is happening, should be investigated with the judged guilty parties being punished according to the laws of their country.
Tax avoidance is very different than tax evasion. I avoid a lot of taxes by donating to charity. (I donate more than I can deduct actually. I do not donate for tax avoidance purposes, that is just an added benefit.) By structuring my income from capital gains (not illegal structuring) and making use of a limited liability corporation I am able to reduce the amount of taxes owed in a lawful manner which is ethical. That is perfectly legal and is something that one probably should do. Now if I have money in a safe deposit box in the Cayman Islands and I have a currier bring it to me in $9500 increments while also lugging a bunch of pre-paid debit cards in his suitcase then I am evading taxes and that is illegal and immoral.
To clarify my own definitions: Morals do not change (though are not absolutely universal). Ethics are mutable, they can be changed for the situation. Cf. Situational Ethics.