No one is forced to join a union. However, a scab will be required to pay 85% of the standard union dues in the form of a negotiations fee as return for the union wage he is earning, a wage which the union obtained at its own expense.
What "Right to Work" really amounts to is giving the scab the right to collect a union wage without paying the negotiations fee, thus garnering a significantly higher wage for the scab than the dues-paying union member. Obviously, the intent of so-called "Right to Work" is to encourage union members to jump ship and join the scabs by giving a financial incentive to do so.
"Right to Work" is just a veiled method of union-busting.