That's not what it meant at all. People often leave out the time period in which this was said. Slapping someone with the back of your hand was an act that was down from a superior to an inferior, while you only slapped someone with the open palm of your hand if you acknowledged them as your equal. So when you get back handed, you turn the other cheek, daring them to slap you again, this time with their palm, and acknowledge you as an equal. It was a way for a slave to show defiance.

Here's another. Jesus said if you have to carry a soldier's gear for a mile, carry it for two. Again, he's speaking to those that were slaves. What is the meaning of this though? Well, the soldiers were allowed to have their slaves carry their gear for 1 mile, but not more because the Romans believed that more than that would make the soldier soft. So you find yourself a slave to one of these soldiers and he orders you to carry his pack. You carry it and the hour ends, but instead of giving it back, you keep on carrying it. Then the soldier's officer notices this and the soldier gets his ass reamed for defying orders.

Jesus taught us methods of fighting back against oppressors when we find ourselves without power. He did not teach us to be weak.