A couple reasons, one is that Amazon made that weird denial tweet with "You don’t really believe the peeing in bottles thing, do you?", denying that their drivers ever peed in bottles. Most companies just put out a statement "We're looking for ways to improve etc etc...", so it was bizarre enough that the meme has carried forward. Amazon has a 1.75T market cap and is reporting profits in the many billions, so people are looking for them to start humanizing their jobs.

UPS drivers are, like you said, part of a union and if it was an issue, they have bargaining power and could get it addressed.

I think it's just a thing that sounds gross to anyone who hasn't tried it, but it's actually very convenient. If I were given a 20 minute break to drive to a restroom, I'd pee in a bottle and take a 19 minute break from driving.

Amazon gets singled out because they are one of the largest, most powerful companies in the history of mankind, by almost any measure: market cap, revenue, employees, etc.

UPS is not.

Amazon wields immense power over multiple industries, and they are scrutinized as such.

Amazon sets the standard that other copy and Amazon actually has the funds to fix it but would rather add another point to their stock price.