I've witnessed several cases where two or more persons fall victim of plagiarism but end up having different penalties for the same crime committed, the newbies are sent home while the rules are somehow bent to accommodate the established members. Rules should be rules.
Even though I agree with you that rules should be the same for everyone, you have to understand that each moderator has his own interpretation of the rules and hence uses his own judgement. The ability to follow their own interpretation is actually one of the rules. That's why the admins can be a bit more lenient towards established members if getting rid of them would present a loss for the whole forum. The rules are unofficial and not something that is set in stone.
It might seem unfair, but it's OK if a distinction is made between the violations of established members and newbies who haven't contributed anything to the forum and are only here to abuse it. When it comes to plagiarism, each case is different. Most of the times, members get banned for it, other times only the post is deleted with no harder punishment for the member.