You have to say to wich database you want to connect. Because sqlplus is not asking for a db-name you have to add it behind the username.

That is when your Oracle environment has not been set.

If you export $ORACLE_SID=[sid] on a Unix OS before you start sqlplus you don't have to fill in the sid any more.

So some unix users have added the export $ORACLE_SID=[sid] in there .profile. So that can explain why some of your users don't have to add the @sid...