@Sp0nge: The reason, I believe, why those servers are whitelisted simply is that it adds a buffer to login in. Minecraft clients that are modified for griefing will offer the option of login in with whatever name the user chooses, simple cracked clients will probably also offer this option.
Without a whitelist, griefers or trolls can be banned by the mods/admins of the respective server, but they can very simply just login again under a different name. The mods and admins then have the option to ban their IP or even an entire IP-range, but even this can be countered by using dynamic IPs.
A whitelist now makes this harder as of course griefers here can do the same as above - they will still have to apply again though, and wait for being whitelisted.