Plagiarism: you're stealing someone else works and passing it as yours, Scamming: you're stealing someone else assets. So they're basically the same thing but just as others have said, one has to do with the forum since they have a rule against plagiarism but when it comes to scams the forum let the community to handle that, that's why it doesn't come with a ban. Plagiarism can be easily handled by the forum since it's easier to catch the defaulters but if the forum wants to tackle scams that'll be a challenge to have a rule for that since we have numerous type of scams and new ones are been invented steadily so it's safer to let the community handle that.

Also having your account ban or red tag both make your account useless so neither the scammer or plagiarist are left unpunished.
Ps: if a plagiarist gets just a red tagged, he might continue in that habit and keep benefiting from someone else hustle which defeats the whole point of the rule in the first place but with the scammer, his trick would have been exposed so people would avoid dealing with that particular account that has been marked for scamming.