When I first heard about this I thought it was funny. Then members were sending me links and I thought they were messing with me. Then I realized they were serious. I am still in a state of shock.
The eruv (a rabbinic enactment) only works in places that carrying is only prohibited by rabbinic enactment. The two laws were made by the same source. The purpose of the rabbinic prohibition to carry is to keep people cognizant of the biblical prohibition to carry in a biblical public thoroughfare. By requiring people to have an eruv to carry there, it would keep awareness up of the prohibition to carry.
(The prohibition wasn't first created, and then the eruv as a workaround. The enactment was a prohibition to carry in places lacking an eruv. The purpose was to distance people from carrying in a place which is biblically prohibited, which an eruv does not help for.)