serial codes and online games

[ilikestragegieshooter]

i am just curious...

you can use this serial /key generators for various games, but the minute you want to log in or register for online gaming, the key doesn`t work...

joint operation, battlefield vietnam/1942 for example.

i really would like to know, how you can pass with this key generators the copy protection but then fail on the online gaming ... how can they find out that this is not a real key?

maybe some of your professionals can enlighten me?!