People shouldn’t have to choose between joining a union or not being able to work at a job they were offered. Unions spend too much money on political activism, and forcing a worker to fund activities they don’t agree with through dues is unethical.
Prospective employees already have a choice -- take the job and join a union or find work elsewhere. States with right-to-work laws tend to have lower wages than states without them, so why would we want this on the federal level?