The name derives from the belief that carpetbaggers put all their belongings in a cheap carpet bag and headed south.
They are not to be confused with scalawags, who were southerners who were Republican sympathizers.
Certainly many carpetbaggers were corrupt, and the word forever has the meaning of "an outsider who moves someplace to exploit the natives and enrich himself at their expense."
UK usage
The term has been used in since the 1990s in the United Kingdom to refer to people who become members of mutually owned societies - such as Building societies - in the hope of gaining shares should they demutualise.