How do exchange services provide forked coins to their customers for free? I know we can get forked coins by donating to ICO teams but I don't think the exchange services donte to them.
So I really consider how to get forked coins for free
Forked coins have nothing to do with ICOs.

You get forked coins by holding the coin that was forked from before the fork. After the fork, you will have the same amount of the original coin and the same amount of the forked coin. Exchanges who hold coins that then get forked will then also get the same amount of forked coins, as does anyone else. Whether they give you access to these forked coins is up to the exchange. 

It may be technically difficult for them to do so so an exchange might not give you access to the forked coins. They may wish to keep the coins to themselves instead of giving free money to their customers. Without asking the exchange themselves, you cannot know why they didn't give you the forked coins.