People aren't complaining about it's mere existence.

They're complaining about the fact that its presence inherently implies an expectation to tip. And empathic people that are patronizing a merchant that has any amount of service provided will, when blatantly presented the option to tip, feel obligated to, especially since the person that will be getting the tip is typically staying at them while they complete the process.

Now you can say something like "get over it", but lots of people don't like being made to feel like that. Even the ones that do still put their foot down and refuse still don't enjoy being asked.

I would venture to guess that even people who like tipping enjoy it more when it's not an obvious option, because offering one at all makes it feel like a grander gesture.

The option to tip has always existed, but making it more streamlined and impossible to auto skip it is just uncomfortable. Not choosing to and intentionally choosing not to are different things, and it makes people uncomfortable.