What specifically makes them underpaid? Are their wages getting stolen or illegally withheld? Is there some market force that is coercing lower wages than they would get otherwise?
According to the Legal Defense Foundation, right-to-work laws prohibit union security agreements, or agreements between employers and labor unions, that govern the extent to which an established union can require employees' membership, payment of union dues, or fees as a condition of employment, either before or after hiring. Right-to-work laws do not aim to provide general guarantee of employment to people seeking work, but rather are a government regulation of the contractual agreements between employers and labor unions that prevents them from excluding non-union workers,[2] or requiring employees to pay a fee to unions that have negotiated the labor contract all the employees work under.
https://en.wikipedia.org/wiki/Right-to-work_law
As noted elsewhere in this subthread, UPS by contrast is unionized (or used to be or is in parts) and seems to offer a livable wage to its drivers.
Whereas the Gig Economy rationalization seems to be: "Show me the gun that was held to your head when you accepted this lousy job."
These concerns don't persuade me personally that Right to Work laws are bad. I'm just telling you they exist and probably have some validity.
I may have misunderstood what you meant though.
https://en.wikipedia.org/wiki/The_Logic_of_Collective_Action
The doublespeak "Right to Work" sabotages collective bargaining, disempowering Labor.
And exploiting desperate people can lead you down a dark path as a society... provided anyone in the US still gives a shit about empathy, morality and the value of life.