The issue is not that the employee is being underpaid (although that is a problem), it's that the underpayment is only possible because the government (i.e. taxpayers) is also giving money to that employee. If that safety net weren't in place then Amazon wouldn't be able to employee these people at these rates since it wouldn't be enough for people to survive on. Essentially, the government is subsidizing Amazon.

Also I'm not sure that someone accepting a specific salary or hourly wage is evidence that they thought it was fair. Employers (especially ones as large as Amazon) can afford to wait far longer for employees than most employees can afford to wait for an employer. I certainly don't think getting paid $20k a year to be a programmer would be fair, but if there no other jobs available to me at that rate then I would be forced to accept it, since I wouldn't survive otherwise.

And if I made $7.25 an hour, I highly doubt I would be able to build up a financial cushion large enough to allow me to quit a job because it didn't pay enough. Even a couple weeks without working probably isn't feasible for somebody trying to support children on $7.25. Leaving a job because it's underpaid and taking the time to find a better one is feasible for professionals, but almost definitely not for many service workers.