Earlier quoted context omitted.
Three reasons. (1) I can afford it. (2) The increase in price and latency will probably result in my ordering less stuff. (3) I doubt that any company is going to allocate a substantial portion of their profits without raising prices. If it’s between raising prices, which I can afford, and people who do difficult jobs I depend on not having an easier, fairer life, then I’ll choose the raised prices.
UPS has a lot of fixed costs and doing less business could hurt them more than making less profits. They have to start paying higher wages now because of the deal they made. Their customers likely can't absorb higher prices as quickly and like you point out will cause a decrease in business as people ship less. It's a balance for UPS and it very likely isn't "just raise prices to match".
I can only control the part of the transaction that originates from me. My commitment is to seek out the solution that seems, to my level of understanding and investigation, to result in the fairest treatment of workers given my lack of price sensitivity.
To do anything less is to give in to capitalist nihilism. It's tempting, but I'm not there yet, personally.