No one is forced to work at UPS; a certain competing and non-union package delivery company also hires seasonal workers, too.
Union dues for seasonal workers is the cost of getting a job there. Just like the taxes that are deducted from our paychecks; people complain about government "stealing" our money, when it's nothing more than the cost of working (on the books) in the United States and one's respective state, even if one is never affected (very unlikely someone is never affected) or has to use the services said deductions pay for. Don't like paying taxes? Work in a different state or country. Don't like paying dues? Work for a non-unionized company. But one chooses where they work, whether it be a union shop that collects dues, or a specific state or nation that imposes taxes to support the system that the companies and employees take advantage of to make the money they do. If the driver helpers think the net pay isn't worth it, don't take the job.